Implement server-enabled Yandex ID auth

Signed-off-by: denis0001-dev <denis0001.dev@ya.ru>

app/android/src/main/AndroidManifest.xml

@@ -42,6 +42,15 @@
                     android:host="u"
                     android:pathPrefix="/" />
             </intent-filter>
+            <intent-filter>
+                <action android:name="android.intent.action.VIEW" />
+                <category android:name="android.intent.category.DEFAULT" />
+                <category android:name="android.intent.category.BROWSABLE" />
+                <data
+                    android:scheme="fromchat"
+                    android:host="oauth"
+                    android:pathPrefix="/yandex" />
+            </intent-filter>
         </activity>
         <service
             android:name=".fcm.FromChatFirebaseMessagingService"

app/android/src/main/kotlin/ru/fromchat/MainActivity.kt

@@ -228,6 +228,7 @@ class MainActivity : ComponentActivity() {
     }
 
     override fun onCreate(savedInstanceState: Bundle?) {
+        Logger.i("MainActivity", "onCreate savedInstanceStateNull=${savedInstanceState == null}")
         super.onCreate(savedInstanceState)
         installSplashScreen()
         enableEdgeToEdge()
@@ -264,13 +265,25 @@ class MainActivity : ComponentActivity() {
     }
 
     override fun onPause() {
+        Logger.i("MainActivity", "onPause")
         super.onPause()
         prevIsPublicChatVisible = isPublicChatVisible
         isPublicChatVisible = false
     }
 
     override fun onResume() {
+        Logger.i("MainActivity", "onResume")
         super.onResume()
         isPublicChatVisible = prevIsPublicChatVisible ?: false
     }
-}
+
+    override fun onDestroy() {
+        Logger.i("MainActivity", "onDestroy isFinishing=$isFinishing")
+        super.onDestroy()
+    }
+
+    override fun onSaveInstanceState(outState: Bundle) {
+        Logger.i("MainActivity", "onSaveInstanceState")
+        super.onSaveInstanceState(outState)
+    }
+}

app/shared/build.gradle.kts

@@ -94,6 +94,7 @@ kotlin {
             implementation(libs.sqldelight.driver.android)
             implementation(libs.livekit.android)
             implementation(libs.livekit.android.compose.components)
+            implementation(libs.androidx.webkit)
         }
 
         iosMain.dependencies {

app/shared/src/androidMain/kotlin/ru/fromchat/ui/auth/yandex/YandexOAuthWebView.android.kt

@@ -0,0 +1,706 @@
+package ru.fromchat.ui.auth.yandex
+
+import android.annotation.SuppressLint
+import android.app.Activity
+import android.content.Context
+import android.content.ContextWrapper
+import android.content.Intent
+import android.graphics.Bitmap
+import android.graphics.Color as AndroidColor
+import android.graphics.Rect
+import android.net.Uri
+import android.os.Build
+import android.os.Bundle
+import android.os.Handler
+import android.os.Looper
+import android.view.PixelCopy
+import android.webkit.CookieManager
+import android.webkit.WebChromeClient
+import android.webkit.WebResourceRequest
+import android.webkit.WebView
+import android.webkit.WebViewClient
+import androidx.compose.animation.AnimatedVisibility
+import androidx.compose.animation.core.tween
+import androidx.compose.animation.fadeIn
+import androidx.compose.animation.fadeOut
+import androidx.compose.foundation.background
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.material3.ExperimentalMaterial3Api
+import androidx.compose.material3.ExperimentalMaterial3ExpressiveApi
+import androidx.compose.material3.LoadingIndicator
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.DisposableEffect
+import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.SideEffect
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.rememberUpdatedState
+import androidx.compose.runtime.saveable.rememberSaveable
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.graphics.luminance
+import androidx.compose.ui.graphics.toArgb
+import androidx.compose.ui.platform.LocalView
+import androidx.compose.ui.viewinterop.AndroidView
+import androidx.core.view.WindowCompat
+import androidx.lifecycle.Lifecycle
+import androidx.lifecycle.LifecycleEventObserver
+import androidx.lifecycle.compose.LocalLifecycleOwner
+import androidx.lifecycle.repeatOnLifecycle
+import androidx.webkit.WebSettingsCompat
+import androidx.webkit.WebViewFeature
+import kotlinx.coroutines.delay
+import kotlinx.coroutines.isActive
+import kotlinx.coroutines.suspendCancellableCoroutine
+import ru.fromchat.Logger
+import ru.fromchat.auth.yandex.YANDEX_OAUTH_REDIRECT_URI
+import ru.fromchat.auth.yandex.extractOAuthCode
+import ru.fromchat.ui.components.PredictiveBackHandler
+import kotlin.coroutines.resume
+
+private const val LOG_TAG = "YandexOAuthWV"
+private const val LOADING_FADE_MS = 250
+private const val LOADING_HIDE_DELAY_MS = 500L
+private const val TOP_ROW_SAMPLE_INTERVAL_MS = 50L
+/** View tag: last applied [darkTheme] so we do not re-set force-dark (that reloads the page). */
+private const val TAG_APPLIED_DARK_THEME = 0x46C7_0A01
+
+private fun shortUrl(url: String?): String {
+    if (url.isNullOrBlank()) return "null"
+    return if (url.length <= 120) url else url.take(117) + "..."
+}
+
+private const val FORCE_COLOR_SCHEME_JS = """
+(function(scheme) {
+  try {
+    var meta = document.querySelector('meta[name="color-scheme"]');
+    if (!meta) {
+      meta = document.createElement('meta');
+      meta.name = 'color-scheme';
+      (document.head || document.documentElement).appendChild(meta);
+    }
+    meta.content = scheme;
+    document.documentElement.style.colorScheme = scheme;
+    document.documentElement.setAttribute('data-theme', scheme);
+    document.documentElement.classList.toggle('theme_dark', scheme === 'dark');
+    document.documentElement.classList.toggle('Theme_color_dark', scheme === 'dark');
+    document.documentElement.classList.toggle('theme_light', scheme === 'light');
+    try { localStorage.setItem('color-scheme', scheme); } catch (e) {}
+    try { localStorage.setItem('theme', scheme); } catch (e) {}
+  } catch (e) {}
+})
+"""
+
+private const val DISABLE_USER_SELECT_JS = """
+(function() {
+  try {
+    var id = 'fromchat-no-select';
+    if (document.getElementById(id)) return;
+    var s = document.createElement('style');
+    s.id = id;
+    s.textContent = [
+      '*,*::before,*::after{',
+      '-webkit-user-select:none!important;',
+      'user-select:none!important;',
+      '-webkit-touch-callout:none!important;',
+      '}',
+      'input,textarea,select,[contenteditable],[contenteditable="true"],',
+      'input *,textarea *,[contenteditable] *,[contenteditable="true"] *{',
+      '-webkit-user-select:text!important;',
+      'user-select:text!important;',
+      '-webkit-touch-callout:default!important;',
+      '}'
+    ].join('');
+    (document.head || document.documentElement).appendChild(s);
+  } catch (e) {}
+})();
+"""
+
+/** Document scroll offset (not just window); >0 means the page start is off-screen. */
+private const val PAGE_SCROLL_Y_JS = """
+(function() {
+  var y = window.scrollY || document.documentElement.scrollTop || document.body.scrollTop || 0;
+  var nodes = document.querySelectorAll('html,body,main,#root,#app,[data-scroll],.Scroll,.scroll,.passp-page');
+  for (var i = 0; i < nodes.length; i++) {
+    try { y = Math.max(y, nodes[i].scrollTop || 0); } catch (e) {}
+  }
+  return y;
+})();
+"""
+
+@OptIn(ExperimentalMaterial3Api::class, ExperimentalMaterial3ExpressiveApi::class)
+@SuppressLint("SetJavaScriptEnabled")
+@Composable
+actual fun YandexOAuthWebView(
+    authorizeUrl: String,
+    languageTag: String,
+    darkTheme: Boolean,
+    fallbackColor: Color,
+    clearCookies: Boolean,
+    onPageBackgroundColor: (Color) -> Unit,
+    onHistoryBackAvailabilityChanged: (Boolean) -> Unit,
+    onCode: (String) -> Unit,
+    onError: (String) -> Unit,
+    onCancel: () -> Unit,
+) {
+    var webView by remember { mutableStateOf<WebView?>(null) }
+    var canGoBack by remember { mutableStateOf(false) }
+    var pageLoading by remember { mutableStateOf(true) }
+    var showLoadingOverlay by remember { mutableStateOf(true) }
+    var chromeColor by remember { mutableStateOf(fallbackColor) }
+    // Survives Activity recreate so the OAuth page is not reloaded from authorizeUrl.
+    val savedWebViewState = rememberSaveable { Bundle() }
+    val lang = languageTag.substringBefore('-').lowercase().ifBlank { "en" }
+    val scheme = if (darkTheme) "dark" else "light"
+    val onHistoryBackAvailabilityChangedState = rememberUpdatedState(onHistoryBackAvailabilityChanged)
+    val onPageBackgroundColorState = rememberUpdatedState(onPageBackgroundColor)
+    val onCodeState = rememberUpdatedState(onCode)
+    val onErrorState = rememberUpdatedState(onError)
+    val onCancelState = rememberUpdatedState(onCancel)
+    val lifecycleOwner = LocalLifecycleOwner.current
+    val instanceId = remember { Integer.toHexString(System.identityHashCode(Any())) }
+
+    DisposableEffect(instanceId) {
+        Logger.i(
+            LOG_TAG,
+            "compose enter id=$instanceId darkTheme=$darkTheme lang=$lang " +
+                "savedBundleEmpty=${savedWebViewState.isEmpty} " +
+                "savedBundleSize=${savedWebViewState.size()} " +
+                "lifecycle=${lifecycleOwner.lifecycle.currentState} " +
+                "authorizeUrl=${shortUrl(authorizeUrl)}",
+        )
+        onDispose {
+            Logger.i(LOG_TAG, "compose dispose id=$instanceId")
+        }
+    }
+
+    fun applyChromeColor(color: Color) {
+        if (chromeColor == color) return
+        chromeColor = color
+        onPageBackgroundColorState.value(color)
+        webView?.setBackgroundColor(color.toArgb())
+    }
+
+    fun updateCanGoBack(value: Boolean) {
+        if (canGoBack != value) {
+            Logger.d(LOG_TAG, "canGoBack $canGoBack → $value id=$instanceId")
+            canGoBack = value
+            onHistoryBackAvailabilityChangedState.value(value)
+        }
+    }
+
+    ApplyYandexWebViewSystemBars(
+        chromeColor = chromeColor,
+        darkTheme = darkTheme,
+        restoreSurfaceColor = fallbackColor,
+    )
+
+    LaunchedEffect(pageLoading) {
+        Logger.d(LOG_TAG, "pageLoading=$pageLoading → overlay scheduling id=$instanceId")
+        if (pageLoading) {
+            showLoadingOverlay = true
+        } else {
+            delay(LOADING_HIDE_DELAY_MS)
+            showLoadingOverlay = false
+        }
+    }
+
+    LaunchedEffect(webView, lifecycleOwner) {
+        val wv = webView ?: return@LaunchedEffect
+        // PixelCopy crashes if the window surface is gone (pause/stop). Only sample while resumed.
+        lifecycleOwner.lifecycle.repeatOnLifecycle(Lifecycle.State.RESUMED) {
+            Logger.d(LOG_TAG, "color sample loop start id=$instanceId")
+            while (isActive) {
+                // Only sample when the document top is visible — not the current scrolled viewport.
+                if (pageScrollY(wv) <= 1.0) {
+                    sampleTopRowColor(wv)?.let { applyChromeColor(it) }
+                }
+                delay(TOP_ROW_SAMPLE_INTERVAL_MS)
+            }
+        }
+        Logger.d(LOG_TAG, "color sample loop paused (not resumed) id=$instanceId")
+    }
+
+    DisposableEffect(webView, lifecycleOwner) {
+        val wv = webView
+        if (wv == null) {
+            return@DisposableEffect onDispose { }
+        }
+        val observer = LifecycleEventObserver { _, event ->
+            Logger.i(
+                LOG_TAG,
+                "lifecycle $event id=$instanceId wv=${Integer.toHexString(System.identityHashCode(wv))} " +
+                    "url=${shortUrl(wv.url)} progress=${wv.progress} canGoBack=${wv.canGoBack()}",
+            )
+            when (event) {
+                Lifecycle.Event.ON_PAUSE -> wv.onPause()
+                Lifecycle.Event.ON_RESUME -> wv.onResume()
+                else -> Unit
+            }
+        }
+        lifecycleOwner.lifecycle.addObserver(observer)
+        if (lifecycleOwner.lifecycle.currentState.isAtLeast(Lifecycle.State.RESUMED)) {
+            wv.onResume()
+        }
+        onDispose {
+            Logger.d(LOG_TAG, "lifecycle observer dispose id=$instanceId")
+            lifecycleOwner.lifecycle.removeObserver(observer)
+            wv.onPause()
+        }
+    }
+
+    // Always consume predictive back here. If NavHost owns the gesture it scales this
+    // screen and WebView jumps scroll-to-top + leaves a gap under the content.
+    PredictiveBackHandler(
+        enabled = true,
+        onProgress = { },
+        onCommit = {
+            val wv = webView
+            if (wv != null && wv.canGoBack()) {
+                wv.goBack()
+            } else {
+                onCancelState.value()
+            }
+        },
+        onCancel = { },
+    )
+
+    val client = remember(authorizeUrl, lang, darkTheme) {
+        Logger.i(
+            LOG_TAG,
+            "WebViewClient create id=$instanceId darkTheme=$darkTheme lang=$lang " +
+                "authorizeUrl=${shortUrl(authorizeUrl)}",
+        )
+        object : WebViewClient() {
+            private fun handleSpecialUrl(view: WebView?, url: String?): Boolean {
+                if (url.isNullOrBlank()) return false
+                if (url.startsWith("fromchat://", ignoreCase = true)) {
+                    Logger.i(LOG_TAG, "intercept redirect url=${shortUrl(url)} id=$instanceId")
+                    val code = extractOAuthCode(url)
+                    if (code != null) {
+                        onCodeState.value(code)
+                    } else {
+                        onErrorState.value("")
+                    }
+                    return true
+                }
+                if (!isYandexAuthNavigation(url)) {
+                    Logger.d(LOG_TAG, "external nav url=${shortUrl(url)} id=$instanceId")
+                    view?.context?.let { ctx ->
+                        runCatching {
+                            ctx.startActivity(
+                                Intent(Intent.ACTION_VIEW, Uri.parse(url)).apply {
+                                    addFlags(Intent.FLAG_ACTIVITY_NEW_TASK)
+                                },
+                            )
+                        }
+                    }
+                    return true
+                }
+                return false
+            }
+
+            private fun applyPageChrome(view: WebView?) {
+                view ?: return
+                view.evaluateJavascript("$FORCE_COLOR_SCHEME_JS('$scheme');", null)
+                view.evaluateJavascript(DISABLE_USER_SELECT_JS, null)
+            }
+
+            override fun shouldOverrideUrlLoading(view: WebView?, request: WebResourceRequest?): Boolean {
+                val url = request?.url?.toString() ?: return false
+                return handleSpecialUrl(view, url)
+            }
+
+            @Deprecated("Deprecated in Java")
+            override fun shouldOverrideUrlLoading(view: WebView?, url: String?): Boolean {
+                return handleSpecialUrl(view, url)
+            }
+
+            override fun onPageStarted(view: WebView?, url: String?, favicon: Bitmap?) {
+                Logger.i(
+                    LOG_TAG,
+                    "onPageStarted id=$instanceId url=${shortUrl(url)} " +
+                        "wv=${view?.let { Integer.toHexString(System.identityHashCode(it)) }} " +
+                        "canGoBack=${view?.canGoBack()} stack=${Throwable().stackTraceToString().lineSequence().take(8).joinToString(" ← ")}",
+                )
+                if (url != null && url.startsWith(YANDEX_OAUTH_REDIRECT_URI, ignoreCase = true)) {
+                    handleSpecialUrl(view, url)
+                }
+                pageLoading = true
+                applyPageChrome(view)
+                updateCanGoBack(view?.canGoBack() == true)
+            }
+
+            override fun onPageFinished(view: WebView?, url: String?) {
+                Logger.i(
+                    LOG_TAG,
+                    "onPageFinished id=$instanceId url=${shortUrl(url)} " +
+                        "progress=${view?.progress} canGoBack=${view?.canGoBack()} " +
+                        "historySize=${view?.copyBackForwardList()?.size}",
+                )
+                pageLoading = false
+                applyPageChrome(view)
+                updateCanGoBack(view?.canGoBack() == true)
+            }
+
+            override fun doUpdateVisitedHistory(view: WebView?, url: String?, isReload: Boolean) {
+                Logger.d(
+                    LOG_TAG,
+                    "doUpdateVisitedHistory id=$instanceId isReload=$isReload url=${shortUrl(url)} " +
+                        "canGoBack=${view?.canGoBack()}",
+                )
+                updateCanGoBack(view?.canGoBack() == true)
+            }
+        }
+    }
+
+    Box(
+        modifier = Modifier
+            .fillMaxSize()
+            .background(chromeColor),
+    ) {
+        AndroidView(
+            factory = { context ->
+                val activity = context.findActivity() ?: context
+                Logger.i(
+                    LOG_TAG,
+                    "AndroidView.factory START id=$instanceId " +
+                        "savedEmpty=${savedWebViewState.isEmpty} savedSize=${savedWebViewState.size()} " +
+                        "darkTheme=$darkTheme clearCookies=$clearCookies " +
+                        "stack=${Throwable().stackTraceToString().lineSequence().drop(1).take(10).joinToString(" ← ")}",
+                )
+                if (clearCookies) {
+                    clearYandexWebViewCookies()
+                }
+                seedYandexThemeCookies(darkTheme)
+                WebView(activity).apply {
+                    setBackgroundColor(fallbackColor.toArgb())
+                    settings.javaScriptEnabled = true
+                    settings.domStorageEnabled = true
+                    settings.javaScriptCanOpenWindowsAutomatically = true
+                    settings.setSupportMultipleWindows(true)
+                    applyYandexDarkSettingsIfNeeded(this, darkTheme)
+                    webViewClient = client
+                    webChromeClient = object : WebChromeClient() {
+                        override fun onCreateWindow(
+                            view: WebView?,
+                            isDialog: Boolean,
+                            isUserGesture: Boolean,
+                            resultMsg: android.os.Message?,
+                        ): Boolean {
+                            val transport = resultMsg?.obj as? WebView.WebViewTransport ?: return false
+                            val temp = WebView(activity).apply {
+                                applyYandexDarkSettingsIfNeeded(this, darkTheme)
+                                webViewClient = object : WebViewClient() {
+                                    override fun shouldOverrideUrlLoading(
+                                        v: WebView?,
+                                        request: WebResourceRequest?,
+                                    ): Boolean {
+                                        val url = request?.url?.toString() ?: return false
+                                        if (!isYandexAuthNavigation(url)) {
+                                            runCatching {
+                                                context.startActivity(
+                                                    Intent(Intent.ACTION_VIEW, Uri.parse(url)),
+                                                )
+                                            }
+                                        } else {
+                                            view?.loadUrl(url)
+                                        }
+                                        return true
+                                    }
+                                }
+                            }
+                            transport.webView = temp
+                            resultMsg.sendToTarget()
+                            return true
+                        }
+                    }
+                    val hadSavedState = savedWebViewState.isEmpty.not()
+                    val restoreList = if (hadSavedState) restoreState(savedWebViewState) else null
+                    val historySize = copyBackForwardList().size
+                    val restored = hadSavedState && restoreList != null && historySize > 0
+                    Logger.i(
+                        LOG_TAG,
+                        "AndroidView.factory restore id=$instanceId hadSaved=$hadSavedState " +
+                            "restoreListNull=${restoreList == null} historySize=$historySize " +
+                            "restored=$restored wv=${Integer.toHexString(System.identityHashCode(this))}",
+                    )
+                    if (restored) {
+                        pageLoading = false
+                        showLoadingOverlay = false
+                        updateCanGoBack(canGoBack())
+                    } else {
+                        savedWebViewState.clear()
+                        Logger.w(
+                            LOG_TAG,
+                            "AndroidView.factory loadUrl (RESET) id=$instanceId url=${shortUrl(authorizeUrl)}",
+                        )
+                        loadUrl(
+                            authorizeUrl,
+                            mapOf("Accept-Language" to "$languageTag,$lang;q=0.9,en;q=0.8"),
+                        )
+                    }
+                    webView = this
+                }
+            },
+            modifier = Modifier.fillMaxSize(),
+            update = { wv ->
+                val clientChanged = wv.webViewClient !== client
+                val darkBefore = wv.getTag(TAG_APPLIED_DARK_THEME) as? Boolean
+                applyYandexDarkSettingsIfNeeded(wv, darkTheme)
+                if (clientChanged) {
+                    Logger.i(
+                        LOG_TAG,
+                        "AndroidView.update reassign client id=$instanceId " +
+                            "darkBefore=$darkBefore darkTheme=$darkTheme " +
+                            "url=${shortUrl(wv.url)}",
+                    )
+                    wv.webViewClient = client
+                }
+                webView = wv
+                updateCanGoBack(wv.canGoBack())
+            },
+            onRelease = { wv ->
+                Logger.i(
+                    LOG_TAG,
+                    "AndroidView.onRelease id=$instanceId " +
+                        "wv=${Integer.toHexString(System.identityHashCode(wv))} " +
+                        "url=${shortUrl(wv.url)} historySize=${wv.copyBackForwardList().size} " +
+                        "stack=${Throwable().stackTraceToString().lineSequence().drop(1).take(10).joinToString(" ← ")}",
+                )
+                savedWebViewState.clear()
+                wv.saveState(savedWebViewState)
+                Logger.i(
+                    LOG_TAG,
+                    "AndroidView.onRelease saved id=$instanceId " +
+                        "bundleEmpty=${savedWebViewState.isEmpty} bundleSize=${savedWebViewState.size()}",
+                )
+            },
+        )
+
+        AnimatedVisibility(
+            visible = showLoadingOverlay,
+            enter = fadeIn(tween(0)),
+            exit = fadeOut(tween(LOADING_FADE_MS)),
+        ) {
+            Box(
+                modifier = Modifier
+                    .fillMaxSize()
+                    .background(fallbackColor),
+                contentAlignment = Alignment.Center,
+            ) {
+                LoadingIndicator()
+            }
+        }
+    }
+}
+
+@Composable
+private fun ApplyYandexWebViewSystemBars(
+    chromeColor: Color,
+    darkTheme: Boolean,
+    restoreSurfaceColor: Color,
+) {
+    val view = LocalView.current
+    val lightIcons = chromeColor.luminance() > 0.5f
+    val chromeArgb = chromeColor.toArgb()
+    val restoreArgb = restoreSurfaceColor.toArgb()
+
+    // Match page chrome while OAuth is open…
+    SideEffect {
+        val window = (view.context as? Activity)?.window ?: return@SideEffect
+        WindowCompat.setDecorFitsSystemWindows(window, false)
+        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.Q) {
+            window.isNavigationBarContrastEnforced = false
+        }
+        window.decorView.setBackgroundColor(chromeArgb)
+        @Suppress("DEPRECATION")
+        window.statusBarColor = chromeArgb
+        @Suppress("DEPRECATION")
+        window.navigationBarColor = chromeArgb
+        WindowCompat.getInsetsController(window, window.decorView).apply {
+            isAppearanceLightNavigationBars = lightIcons
+            isAppearanceLightStatusBars = lightIcons
+        }
+    }
+
+    // …and restore app theme bars when leaving (SideEffect alone won't re-run if theme deps unchanged).
+    DisposableEffect(darkTheme, restoreArgb) {
+        onDispose {
+            val window = (view.context as? Activity)?.window ?: return@onDispose
+            WindowCompat.getInsetsController(window, view).apply {
+                isAppearanceLightStatusBars = !darkTheme
+                isAppearanceLightNavigationBars = !darkTheme
+            }
+            @Suppress("DEPRECATION")
+            window.statusBarColor = AndroidColor.TRANSPARENT
+            @Suppress("DEPRECATION")
+            window.navigationBarColor = AndroidColor.TRANSPARENT
+            window.decorView.setBackgroundColor(restoreArgb)
+            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.Q) {
+                window.isNavigationBarContrastEnforced = false
+            }
+        }
+    }
+}
+
+private fun Context.findActivity(): Activity? {
+    var ctx: Context? = this
+    while (ctx is ContextWrapper) {
+        if (ctx is Activity) return ctx
+        ctx = ctx.baseContext
+    }
+    return null
+}
+
+private fun clearYandexWebViewCookies() {
+    val cookieManager = CookieManager.getInstance()
+    cookieManager.setAcceptCookie(true)
+    cookieManager.removeAllCookies(null)
+    cookieManager.flush()
+    Logger.i(LOG_TAG, "cleared all WebView cookies for Yandex re-auth")
+}
+
+private fun seedYandexThemeCookies(darkTheme: Boolean) {
+    val theme = if (darkTheme) "dark" else "light"
+    val cookieManager = CookieManager.getInstance()
+    cookieManager.setAcceptCookie(true)
+    val hosts = listOf(
+        "https://yandex.ru",
+        "https://yandex.com",
+        "https://passport.yandex.ru",
+        "https://passport.yandex.com",
+        "https://oauth.yandex.ru",
+        "https://oauth.yandex.com",
+    )
+    for (host in hosts) {
+        cookieManager.setCookie(host, "color_scheme=$theme; path=/")
+        cookieManager.setCookie(host, "theme=$theme; path=/")
+        cookieManager.setCookie(host, "yh=Theme=$theme; path=/")
+    }
+    cookieManager.flush()
+}
+
+/**
+ * Applies force-dark / algorithmic darkening only when [darkTheme] changed.
+ * Re-applying the same force-dark value can reload the page and wipe in-progress OAuth UI.
+ */
+@Suppress("DEPRECATION")
+private fun applyYandexDarkSettingsIfNeeded(webView: WebView, darkTheme: Boolean) {
+    val previous = webView.getTag(TAG_APPLIED_DARK_THEME) as? Boolean
+    if (previous == darkTheme) return
+    Logger.w(
+        LOG_TAG,
+        "applyDarkSettings CHANGE previous=$previous → $darkTheme " +
+            "wv=${Integer.toHexString(System.identityHashCode(webView))} url=${shortUrl(webView.url)}",
+    )
+    webView.setTag(TAG_APPLIED_DARK_THEME, darkTheme)
+    val settings = webView.settings
+    if (WebViewFeature.isFeatureSupported(WebViewFeature.ALGORITHMIC_DARKENING)) {
+        WebSettingsCompat.setAlgorithmicDarkeningAllowed(settings, darkTheme)
+    }
+    if (darkTheme && WebViewFeature.isFeatureSupported(WebViewFeature.FORCE_DARK)) {
+        WebSettingsCompat.setForceDark(settings, WebSettingsCompat.FORCE_DARK_ON)
+    } else if (WebViewFeature.isFeatureSupported(WebViewFeature.FORCE_DARK)) {
+        WebSettingsCompat.setForceDark(settings, WebSettingsCompat.FORCE_DARK_OFF)
+    }
+}
+
+private suspend fun pageScrollY(webView: WebView): Double =
+    suspendCancellableCoroutine { cont ->
+        webView.evaluateJavascript(PAGE_SCROLL_Y_JS) { raw ->
+            val value = raw?.trim()?.removeSurrounding("\"")?.toDoubleOrNull() ?: 0.0
+            cont.resume(value)
+        }
+    }
+
+/**
+ * Samples the first visible pixel row of the WebView (mid-x).
+ * Caller must only invoke this when the document is scrolled to the page start
+ * and the activity is resumed (window still has a surface).
+ */
+private suspend fun sampleTopRowColor(webView: WebView): Color? {
+    if (webView.width <= 0 || webView.height <= 0) return null
+    val activity = webView.context.findActivity() ?: return null
+    if (activity is androidx.lifecycle.LifecycleOwner &&
+        !activity.lifecycle.currentState.isAtLeast(Lifecycle.State.RESUMED)
+    ) {
+        return null
+    }
+    val window = activity.window ?: return null
+    val decor = window.decorView
+    if (!decor.isAttachedToWindow || decor.width <= 0 || decor.height <= 0) return null
+    val loc = IntArray(2)
+    webView.getLocationInWindow(loc)
+    val x = loc[0] + webView.width / 2
+    val y = loc[1]
+    if (x < 0 || y < 0) return null
+    val bitmap = Bitmap.createBitmap(1, 1, Bitmap.Config.ARGB_8888)
+    val src = Rect(x, y, x + 1, y + 1)
+    return suspendCancellableCoroutine { cont ->
+        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.O) {
+            bitmap.recycle()
+            cont.resume(null)
+            return@suspendCancellableCoroutine
+        }
+        try {
+            PixelCopy.request(
+                window,
+                src,
+                bitmap,
+                { result ->
+                    if (result == PixelCopy.SUCCESS) {
+                        val pixel = bitmap.getPixel(0, 0)
+                        cont.resume(Color(pixel))
+                    } else {
+                        cont.resume(null)
+                    }
+                    bitmap.recycle()
+                },
+                Handler(Looper.getMainLooper()),
+            )
+        } catch (e: IllegalArgumentException) {
+            // e.g. "Window doesn't have a backing surface!" after ON_PAUSE/ON_STOP
+            Logger.w(LOG_TAG, "PixelCopy skipped: ${e.message}")
+            bitmap.recycle()
+            cont.resume(null)
+        }
+    }
+}
+
+/**
+ * Keep Yandex ID / OAuth / captcha flows in the WebView; open everything else externally.
+ */
+internal fun isYandexAuthNavigation(url: String): Boolean {
+    if (url.startsWith("fromchat://", ignoreCase = true)) return true
+    val uri = Uri.parse(url)
+    val host = uri.host?.lowercase() ?: return false
+    val path = uri.path.orEmpty().lowercase()
+
+    if (host == "yandex.ru" || host == "www.yandex.ru" || host == "ya.ru" || host == "www.ya.ru") {
+        return path.contains("captcha") ||
+            path.startsWith("/auth") ||
+            path.startsWith("/showcaptcha") ||
+            path.startsWith("/checkcaptcha")
+    }
+
+    return host == "oauth.yandex.com" ||
+        host == "oauth.yandex.ru" ||
+        host.endsWith(".oauth.yandex.com") ||
+        host.endsWith(".oauth.yandex.ru") ||
+        host == "passport.yandex.ru" ||
+        host == "passport.yandex.com" ||
+        host.endsWith(".passport.yandex.ru") ||
+        host.endsWith(".passport.yandex.com") ||
+        host.startsWith("auth.yandex.") ||
+        host.startsWith("login.yandex.") ||
+        host.startsWith("id.yandex.") ||
+        host == "sso.passport.yandex.ru" ||
+        host == "captcha.yandex.net" ||
+        host.endsWith(".captcha.yandex.net") ||
+        (host.contains("captcha") && host.contains("yandex"))
+}

app/shared/src/commonMain/composeResources/drawable/ic_yandex.xml

@@ -0,0 +1,10 @@
+<vector xmlns:android="http://schemas.android.com/apk/res/android"
+    android:width="24dp"
+    android:height="24dp"
+    android:viewportWidth="24"
+    android:viewportHeight="24">
+    <!-- Yandex “Я” mark only; circular brand background removed for tintable icons. -->
+    <path
+        android:fillColor="#FFFFFFFF"
+        android:pathData="M13.32,7.666h-0.924c-1.694,0 -2.585,0.858 -2.585,2.123c0,1.43 0.616,2.1 1.881,2.959l1.045,0.704l-3.003,4.487H7.49l2.695,-4.014c-1.55,-1.111 -2.42,-2.19 -2.42,-4.015c0,-2.288 1.595,-3.85 4.62,-3.85h3.003v11.868H13.32V7.666z" />
+</vector>

app/shared/src/commonMain/composeResources/values-ru/strings.xml

@@ -51,6 +51,12 @@
     <string name="auth_step_password_body">Мы войдём в аккаунт или создадим новый.</string>
     <string name="auth_step_confirm_title">Подтвердите пароль</string>
     <string name="auth_step_confirm_body">Введите тот же пароль ещё раз.</string>
+    <string name="auth_step_yandex_title">Войдите через Яндекс ID</string>
+    <string name="auth_step_yandex_body">Так мы боремся с вредоносными ботами и соблюдаем требования российских законов. Номер телефона нам недоступен — мы видим только имя, фамилию и пол. Эти данные мы не сохраняем — они нужны только для защиты и чтобы заранее заполнить поля на следующем шаге (не бойтесь, вы сможете подправить данные перед отправкой).</string>
+    <string name="auth_step_yandex_cta">Продолжить через Яндекс ID</string>
+    <string name="auth_yandex_webview_title">Яндекс ID</string>
+    <string name="auth_yandex_client_mismatch">Сервер вернул неожиданный идентификатор приложения Яндекса. Обновите приложение или обратитесь в поддержку.</string>
+    <string name="auth_yandex_failed">Вход через Яндекс ID отменён или не удался.</string>
     <string name="chats">Чаты</string>
     <string name="contacts">Контакты</string>
     <string name="profile">Профиль</string>
@@ -418,6 +424,14 @@
     <string name="settings_account_logout_confirm_body">Придётся войти снова.</string>
     <string name="settings_account_delete">Удалить аккаунт</string>
     <string name="settings_account_delete_d">Безвозвратно удалить аккаунт и данные</string>
+    <string name="settings_account_change_yandex">Сменить Яндекс ID</string>
+    <string name="settings_account_change_yandex_d">Привязать другой аккаунт Яндекса</string>
+    <string name="settings_yandex_step_confirm_title">Сменить Яндекс ID?</string>
+    <string name="settings_yandex_step_confirm_body">Это не удаляет ваш аккаунт FromChat. Предыдущий Яндекс ID будет освобождён, а вместо него привяжется тот, с которым вы войдёте.</string>
+    <string name="settings_yandex_step_confirm_cta">Продолжить</string>
+    <string name="settings_yandex_step_done_title">Яндекс ID обновлён</string>
+    <string name="settings_yandex_step_done_body">Аккаунт теперь привязан к Яндекс ID, с которым вы только что вошли.</string>
+    <string name="settings_done">Готово</string>
     <string name="settings_account_delete_confirm_title">Удалить аккаунт?</string>
     <string name="settings_account_delete_confirm_body">Это нельзя отменить.</string>
     <string name="settings_account_deleted">Аккаунт удалён</string>

app/shared/src/commonMain/composeResources/values/strings.xml

@@ -58,6 +58,12 @@
     <string name="auth_step_password_body">We will sign you in or create a new account.</string>
     <string name="auth_step_confirm_title">Confirm your password</string>
     <string name="auth_step_confirm_body">Enter the same password again.</string>
+    <string name="auth_step_yandex_title">Sign in with Yandex ID</string>
+    <string name="auth_step_yandex_body">This helps us fight malicious bots and meet Russian legal requirements. We can’t access your phone number — we only see your first name, last name, and gender. We don’t store that data; it’s only used for protection and to pre-fill the next step (don’t worry — you can edit everything before submitting).</string>
+    <string name="auth_step_yandex_cta">Continue with Yandex ID</string>
+    <string name="auth_yandex_webview_title">Yandex ID</string>
+    <string name="auth_yandex_client_mismatch">This server returned an unexpected Yandex app id. Update the app or contact support.</string>
+    <string name="auth_yandex_failed">Yandex sign-in was cancelled or failed.</string>
     <!-- Main Screen -->
     <string name="chats">Chats</string>
     <string name="contacts">Contacts</string>
@@ -446,6 +452,14 @@
     <string name="settings_account_logout_confirm_body">You will need to sign in again.</string>
     <string name="settings_account_delete">Delete account</string>
     <string name="settings_account_delete_d">Permanently delete your account and data</string>
+    <string name="settings_account_change_yandex">Change Yandex ID</string>
+    <string name="settings_account_change_yandex_d">Link a different Yandex account</string>
+    <string name="settings_yandex_step_confirm_title">Change Yandex ID?</string>
+    <string name="settings_yandex_step_confirm_body">This does not delete your FromChat account. Your previous Yandex ID will be freed, and the new one you sign in with will be linked instead.</string>
+    <string name="settings_yandex_step_confirm_cta">Continue</string>
+    <string name="settings_yandex_step_done_title">Yandex ID updated</string>
+    <string name="settings_yandex_step_done_body">Your account is now linked to the Yandex ID you just signed in with.</string>
+    <string name="settings_done">Done</string>
     <string name="settings_account_delete_confirm_title">Delete account?</string>
     <string name="settings_account_delete_confirm_body">This cannot be undone.</string>
     <string name="settings_account_deleted">Account deleted</string>

app/shared/src/commonMain/kotlin/ru/fromchat/api/ApiClient.kt

@@ -44,7 +44,12 @@ import kotlinx.coroutines.flow.StateFlow
 import kotlinx.coroutines.flow.asStateFlow
 import kotlinx.coroutines.launch
 import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.JsonObject
+import kotlinx.serialization.json.booleanOrNull
+import kotlinx.serialization.json.contentOrNull
+import kotlinx.serialization.json.decodeFromJsonElement
 import kotlinx.serialization.json.encodeToJsonElement
+import kotlinx.serialization.json.jsonPrimitive
 import ru.fromchat.api.ApiClient.logout
 import ru.fromchat.api.ApiClient.persistSessionToStorage
 import ru.fromchat.api.crypto.IdentityKeyManager
@@ -99,11 +104,21 @@ import ru.fromchat.api.schema.user.FcmTokenRequest
 import ru.fromchat.api.schema.user.User
 import ru.fromchat.api.schema.user.UsersSearchResponse
 import ru.fromchat.api.schema.user.VerifyPasswordRequest
+import ru.fromchat.api.schema.user.auth.AuthPasswordStepRequest
+import ru.fromchat.api.schema.user.auth.AuthUsernameStepRequest
+import ru.fromchat.api.schema.user.auth.AuthUsernameStepResponse
+import ru.fromchat.api.schema.user.auth.AccountYandexResponse
+import ru.fromchat.api.schema.user.auth.ChangeYandexRequest
+import ru.fromchat.api.schema.user.auth.ChangeYandexResponse
 import ru.fromchat.api.schema.user.auth.CheckAuthResponse
 import ru.fromchat.api.schema.user.auth.CheckUsernameResponse
 import ru.fromchat.api.schema.user.auth.LoginRequest
 import ru.fromchat.api.schema.user.auth.LoginResponse
+import ru.fromchat.api.schema.user.auth.RegisterConfirmRequest
 import ru.fromchat.api.schema.user.auth.RegisterRequest
+import ru.fromchat.api.schema.user.auth.YandexExchangeRequest
+import ru.fromchat.api.schema.user.auth.YandexExchangeResponse
+import ru.fromchat.api.schema.user.auth.YandexOAuthParams
 import ru.fromchat.api.schema.user.devices.DeviceSessionInfo
 import ru.fromchat.api.schema.user.devices.DevicesListResponse
 import ru.fromchat.api.schema.user.keys.BackupBlobRequest
@@ -485,6 +500,60 @@ object ApiClient {
             }
             .body()
 
+    suspend fun authUsernameStep(username: String): AuthUsernameStepResponse =
+        httpProbe
+            .post("${ServerConfig.apiBaseUrl}/auth/steps/username") {
+                contentType(ContentType.Application.Json)
+                setBody(AuthUsernameStepRequest(username = username.trim()))
+            }
+            .body()
+
+    sealed interface AuthPasswordStepOutcome {
+        data class LoggedIn(val response: LoginResponse) : AuthPasswordStepOutcome
+        data class NeedsRegister(
+            val yandexRequired: Boolean,
+            val yandex: YandexOAuthParams?,
+        ) : AuthPasswordStepOutcome
+    }
+
+    suspend fun authPasswordStep(username: String, passwordDerived: String): AuthPasswordStepOutcome {
+        val raw = httpProbe
+            .post("${ServerConfig.apiBaseUrl}/auth/steps/password") {
+                contentType(ContentType.Application.Json)
+                setBody(
+                    AuthPasswordStepRequest(
+                        username = username.trim(),
+                        password = passwordDerived,
+                    ),
+                )
+            }
+            .body<JsonObject>()
+        val status = raw["status"]?.jsonPrimitive?.contentOrNull
+        return when (status) {
+            "needs_register" -> AuthPasswordStepOutcome.NeedsRegister(
+                yandexRequired = raw["yandex_required"]?.jsonPrimitive?.booleanOrNull == true,
+                yandex = raw["yandex"]?.let { json.decodeFromJsonElement(YandexOAuthParams.serializer(), it) },
+            )
+            else -> AuthPasswordStepOutcome.LoggedIn(json.decodeFromJsonElement(LoginResponse.serializer(), raw))
+        }
+    }
+
+    suspend fun authYandexExchange(code: String, codeVerifier: String): YandexExchangeResponse =
+        httpProbe
+            .post("${ServerConfig.apiBaseUrl}/auth/yandex/exchange") {
+                contentType(ContentType.Application.Json)
+                setBody(YandexExchangeRequest(code = code, code_verifier = codeVerifier))
+            }
+            .body()
+
+    suspend fun authRegisterConfirm(request: RegisterConfirmRequest): LoginResponse =
+        httpProbe
+            .post("${ServerConfig.apiBaseUrl}/auth/steps/register/confirm") {
+                contentType(ContentType.Application.Json)
+                setBody(request)
+            }
+            .body()
+
     /** Public pre-auth lookup; uses probe client so an existing session token is not sent. */
     suspend fun checkUsername(username: String): CheckUsernameResponse =
         httpProbe
@@ -1478,6 +1547,19 @@ object ApiClient {
         }
     }
 
+    suspend fun getAccountYandex(): AccountYandexResponse =
+        http
+            .get("${ServerConfig.apiBaseUrl}/account/yandex")
+            .body()
+
+    suspend fun changeAccountYandex(registrationProof: String): ChangeYandexResponse =
+        http
+            .post("${ServerConfig.apiBaseUrl}/account/yandex") {
+                contentType(ContentType.Application.Json)
+                setBody(ChangeYandexRequest(registration_proof = registrationProof))
+            }
+            .body()
+
     suspend fun verifyPasswordDerived(passwordDerived: String) {
         http.post("${ServerConfig.apiBaseUrl}/verify-password") {
             contentType(ContentType.Application.Json)

app/shared/src/commonMain/kotlin/ru/fromchat/api/schema/user/auth/AuthSteps.kt

@@ -0,0 +1,73 @@
+package ru.fromchat.api.schema.user.auth
+
+import kotlinx.serialization.Serializable
+
+@Serializable
+data class AuthUsernameStepRequest(
+    val username: String,
+)
+
+@Serializable
+data class AuthUsernameStepResponse(
+    val ok: Boolean = true,
+    val exists: Boolean? = null,
+)
+
+@Serializable
+data class AuthPasswordStepRequest(
+    val username: String,
+    val password: String,
+)
+
+@Serializable
+data class YandexOAuthParams(
+    val client_id: String,
+    val redirect_uri: String,
+    val authorize_url: String,
+    val scope: String,
+)
+
+@Serializable
+data class AuthNeedsRegisterResponse(
+    val status: String,
+    val yandex_required: Boolean = false,
+    val yandex: YandexOAuthParams? = null,
+)
+
+@Serializable
+data class YandexExchangeRequest(
+    val code: String,
+    val code_verifier: String,
+)
+
+@Serializable
+data class YandexExchangeResponse(
+    val registration_proof: String,
+)
+
+@Serializable
+data class AccountYandexResponse(
+    val linked: Boolean = false,
+    val yandex: YandexOAuthParams? = null,
+)
+
+@Serializable
+data class ChangeYandexRequest(
+    val registration_proof: String,
+)
+
+@Serializable
+data class ChangeYandexResponse(
+    val status: String? = null,
+    val unchanged: Boolean = false,
+)
+
+@Serializable
+data class RegisterConfirmRequest(
+    val username: String,
+    val display_name: String,
+    val password: String,
+    val confirm_password: String,
+    val bio: String? = null,
+    val registration_proof: String? = null,
+)

app/shared/src/commonMain/kotlin/ru/fromchat/auth/yandex/YandexOAuth.kt

@@ -0,0 +1,166 @@
+package ru.fromchat.auth.yandex
+
+import korlibs.crypto.SHA256
+import kotlin.random.Random
+
+internal data class PkcePair(
+    val codeVerifier: String,
+    val codeChallenge: String,
+)
+
+internal fun generatePkcePair(): PkcePair {
+    val verifierBytes = ByteArray(32).also { Random.Default.nextBytes(it) }
+    val codeVerifier = base64UrlNoPad(verifierBytes)
+    val challenge = base64UrlNoPad(SHA256.digest(codeVerifier.encodeToByteArray()).bytes)
+    return PkcePair(codeVerifier = codeVerifier, codeChallenge = challenge)
+}
+
+private fun base64UrlNoPad(bytes: ByteArray): String {
+    val alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_"
+    val out = StringBuilder((bytes.size * 4 + 2) / 3)
+    var i = 0
+    while (i < bytes.size) {
+        val b0 = bytes[i].toInt() and 0xff
+        val b1 = if (i + 1 < bytes.size) bytes[i + 1].toInt() and 0xff else 0
+        val b2 = if (i + 2 < bytes.size) bytes[i + 2].toInt() and 0xff else 0
+        out.append(alphabet[b0 shr 2])
+        out.append(alphabet[((b0 and 0x03) shl 4) or (b1 shr 4)])
+        if (i + 1 < bytes.size) {
+            out.append(alphabet[((b1 and 0x0f) shl 2) or (b2 shr 6)])
+        }
+        if (i + 2 < bytes.size) {
+            out.append(alphabet[b2 and 0x3f])
+        }
+        i += 3
+    }
+    return out.toString()
+}
+
+internal const val OFFICIAL_API_HOST = "api.fromchat.ru"
+
+/**
+ * Prod Yandex OAuth client id (identity-only app). When non-empty and the API host is
+ * [OFFICIAL_API_HOST], the server-supplied client_id must match this value.
+ */
+internal const val OFFICIAL_YANDEX_OAUTH_CLIENT_ID = ""
+
+internal const val YANDEX_OAUTH_REDIRECT_URI = "fromchat://oauth/yandex"
+
+internal fun isOfficialApiHost(serverIp: String): Boolean {
+    val host = serverIp.trim().lowercase().substringBefore("/").substringBefore(":")
+    return host == OFFICIAL_API_HOST
+}
+
+/**
+ * Returns the client_id to use, or null if the official host sent a mismatched id.
+ */
+internal fun resolveYandexClientId(serverClientId: String, serverIp: String): String? {
+    val trimmed = serverClientId.trim()
+    if (trimmed.isEmpty()) return null
+    if (!isOfficialApiHost(serverIp)) return trimmed
+    val pinned = OFFICIAL_YANDEX_OAUTH_CLIENT_ID.trim()
+    if (pinned.isEmpty()) return trimmed
+    return if (pinned == trimmed) trimmed else null
+}
+
+/**
+ * Yandex UI language follows the authorize host (`.ru` vs `.com`), not OAuth `lang`.
+ * Keep `lang` anyway; rewrite host so passport matches the app locale.
+ */
+internal fun yandexAuthorizeHostForLanguage(languageTag: String): String {
+    val lang = languageTag.substringBefore('-').lowercase().ifBlank { "en" }
+    return if (lang == "ru") "oauth.yandex.ru" else "oauth.yandex.com"
+}
+
+internal fun withYandexAuthorizeHost(authorizeUrl: String, languageTag: String): String {
+    val targetHost = yandexAuthorizeHostForLanguage(languageTag)
+    return Regex("""oauth\.yandex\.(com|ru)""", RegexOption.IGNORE_CASE)
+        .replace(authorizeUrl.trim()) { targetHost }
+}
+
+internal fun buildYandexAuthorizeUrl(
+    authorizeUrl: String,
+    clientId: String,
+    redirectUri: String,
+    scope: String,
+    codeChallenge: String,
+    languageTag: String = "en",
+    darkTheme: Boolean = false,
+): String {
+    val lang = languageTag.substringBefore('-').lowercase().ifBlank { "en" }
+    val base = withYandexAuthorizeHost(authorizeUrl, lang).trimEnd('?')
+    val theme = if (darkTheme) "dark" else "light"
+    val params = buildList {
+        add("response_type" to "code")
+        add("client_id" to clientId)
+        add("redirect_uri" to redirectUri)
+        if (scope.isNotBlank()) {
+            add("scope" to scope.trim())
+        }
+        add("code_challenge" to codeChallenge)
+        add("code_challenge_method" to "S256")
+        add("force_confirm" to "yes")
+        add("lang" to lang)
+        add("theme" to theme)
+        add("color_scheme" to theme)
+    }.joinToString("&") { (k, v) ->
+        "${encodeUrl(k)}=${encodeUrl(v)}"
+    }
+    return "$base?$params"
+}
+
+private fun encodeUrl(value: String): String = buildString(value.length) {
+    for (ch in value) {
+        when {
+            ch.isLetterOrDigit() || ch in "-_.~" -> append(ch)
+            else -> {
+                val bytes = ch.toString().encodeToByteArray()
+                for (b in bytes) {
+                    append('%')
+                    append(((b.toInt() shr 4) and 0xf).toString(16).uppercase())
+                    append((b.toInt() and 0xf).toString(16).uppercase())
+                }
+            }
+        }
+    }
+}
+
+internal fun extractOAuthCode(redirectUrl: String): String? {
+    val uri = redirectUrl.trim()
+    if (!uri.startsWith(YANDEX_OAUTH_REDIRECT_URI, ignoreCase = true) &&
+        !uri.startsWith("fromchat://oauth/yandex?", ignoreCase = true)
+    ) {
+        return null
+    }
+    val query = uri.substringAfter('?', missingDelimiterValue = "")
+    if (query.isEmpty()) return null
+    return query.split('&').firstNotNullOfOrNull { part ->
+        val key = part.substringBefore('=')
+        val raw = part.substringAfter('=', missingDelimiterValue = "")
+        if (key == "code" && raw.isNotEmpty()) decodeUrl(raw) else null
+    }
+}
+
+private fun decodeUrl(value: String): String {
+    val bytes = ArrayList<Byte>()
+    var i = 0
+    while (i < value.length) {
+        val c = value[i]
+        when {
+            c == '+' -> {
+                bytes.add(' '.code.toByte())
+                i++
+            }
+            c == '%' && i + 2 < value.length -> {
+                val hex = value.substring(i + 1, i + 3)
+                bytes.add(hex.toInt(16).toByte())
+                i += 3
+            }
+            else -> {
+                bytes.add(c.code.toByte())
+                i++
+            }
+        }
+    }
+    return bytes.toByteArray().decodeToString()
+}

app/shared/src/commonMain/kotlin/ru/fromchat/ui/App.kt

@@ -79,6 +79,8 @@ import ru.fromchat.legal.DocumentScreen
 import ru.fromchat.legal.DocumentType
 import ru.fromchat.notifications.NotificationLaunchCoordinator
 import ru.fromchat.ui.auth.AuthScreen
+import ru.fromchat.ui.auth.yandex.YandexOAuthNav
+import ru.fromchat.ui.auth.yandex.YandexOAuthScreen
 import ru.fromchat.ui.calls.CallOverlay
 import ru.fromchat.ui.chat.panels.dm.DmChatRoute
 import ru.fromchat.ui.chat.panels.dm.DmNav
@@ -98,6 +100,9 @@ import ru.fromchat.ui.main.settings.NotificationsScreen
 import ru.fromchat.ui.main.settings.SettingsRoutes
 import ru.fromchat.ui.main.settings.account.AccountScreen
 import ru.fromchat.ui.main.settings.account.changepassword.ChangePasswordScreen
+import ru.fromchat.ui.main.settings.account.changeyandex.ChangeYandexConfirmScreen
+import ru.fromchat.ui.main.settings.account.changeyandex.ChangeYandexDoneScreen
+import ru.fromchat.ui.main.settings.account.changeyandex.ChangeYandexOAuthScreen
 import ru.fromchat.ui.main.settings.account.delete.DeleteAccountScreen
 import ru.fromchat.ui.main.settings.server.ServerConfigScreen
 import ru.fromchat.ui.profile.EditProfileFocusField
@@ -457,6 +462,10 @@ fun App(
                                 )
                             }
 
+                            composable(YandexOAuthNav.ROUTE) {
+                                YandexOAuthScreen()
+                            }
+
                             composable("chat") {
                                 MainScreen(
                                     sharedTransitionScope = this@SharedTransitionLayout,
@@ -670,6 +679,26 @@ fun App(
                                 )
                             }
 
+                            settingsComposable(SettingsRoutes.AccountYandexFlow) {
+                                ChangeYandexConfirmScreen(
+                                    onBack = { navController.navigateUp() },
+                                )
+                            }
+
+                            settingsComposable(SettingsRoutes.AccountYandexOAuth) {
+                                ChangeYandexOAuthScreen(
+                                    onBack = { navController.navigateUp() },
+                                )
+                            }
+
+                            settingsComposable(SettingsRoutes.AccountYandexDone) {
+                                ChangeYandexDoneScreen(
+                                    onDone = {
+                                        navController.popBackStack(SettingsRoutes.Account, inclusive = false)
+                                    },
+                                )
+                            }
+
                             settingsComposable(SettingsRoutes.Account) {
                                 AccountScreen(
                                     onBack = { navController.navigateUp() },
@@ -679,6 +708,7 @@ fun App(
                                         }
                                     },
                                     onChangePassword = { navController.navigate(SettingsRoutes.SecurityPasswordFlow) },
+                                    onChangeYandexId = { navController.navigate(SettingsRoutes.AccountYandexFlow) },
                                     onDeleteAccount = { navController.navigate(SettingsRoutes.AccountDeleteFlow) },
                                 )
                             }

app/shared/src/commonMain/kotlin/ru/fromchat/ui/auth/AuthRegisterDraft.kt

@@ -0,0 +1,31 @@
+package ru.fromchat.ui.auth
+
+import ru.fromchat.api.schema.user.auth.YandexOAuthParams
+
+/**
+ * Survives [AuthScreen] leaving composition when navigating to the Yandex OAuth route.
+ * Cleared on welcome / successful auth / explicit reset to username.
+ */
+internal object AuthRegisterDraft {
+    var username: String = ""
+    var password: String = ""
+    var confirmPassword: String = ""
+    var displayName: String = ""
+    var bio: String = ""
+    var yandexRequired: Boolean = false
+    var yandexParams: YandexOAuthParams? = null
+    var registrationProof: String? = null
+    var page: Int = 0
+
+    fun clear() {
+        username = ""
+        password = ""
+        confirmPassword = ""
+        displayName = ""
+        bio = ""
+        yandexRequired = false
+        yandexParams = null
+        registrationProof = null
+        page = 0
+    }
+}

app/shared/src/commonMain/kotlin/ru/fromchat/ui/auth/AuthScreen.kt

@@ -5,6 +5,7 @@ import androidx.compose.material.icons.Icons
 import androidx.compose.material.icons.filled.Storage
 import androidx.compose.material3.SnackbarHostState
 import androidx.compose.runtime.Composable
+import androidx.compose.runtime.DisposableEffect
 import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableStateOf
@@ -27,14 +28,15 @@ import ru.fromchat.api.local.db.clearAccountCacheOnLogout
 import ru.fromchat.api.instance.ServerProbeResult
 import ru.fromchat.api.instance.probeServer
 import ru.fromchat.api.schema.core.ErrorResponse
-import ru.fromchat.api.schema.user.auth.LoginRequest
 import ru.fromchat.api.schema.user.auth.LoginResponse
-import ru.fromchat.api.schema.user.auth.RegisterRequest
+import ru.fromchat.api.schema.user.auth.RegisterConfirmRequest
+import ru.fromchat.api.schema.user.auth.YandexOAuthParams
 import ru.fromchat.change_server
 import ru.fromchat.config.Settings
 import ru.fromchat.ui.LocalNavController
 import ru.fromchat.ui.auth.register.confirmPasswordStepPage
 import ru.fromchat.ui.auth.register.profileStepPage
+import ru.fromchat.ui.auth.yandex.yandexIdStepPage
 import ru.fromchat.ui.components.ExpressiveStepFlowScaffold
 import ru.fromchat.ui.components.Text
 import ru.fromchat.ui.components.TextCta
@@ -47,12 +49,16 @@ private enum class AuthFlowStep {
     Username,
     Password,
     ConfirmPassword,
+    YandexId,
     Profile,
 }
 
 internal sealed interface PasswordStepResult {
     data object LoginSuccess : PasswordStepResult
-    data object AdvanceToRegister : PasswordStepResult
+    data class NeedsRegister(
+        val yandexRequired: Boolean,
+        val yandex: YandexOAuthParams?,
+    ) : PasswordStepResult
     data class WrongPassword(val message: String) : PasswordStepResult
     data class RateLimited(val message: String) : PasswordStepResult
     data class Error(val message: String, val cause: Throwable? = null) : PasswordStepResult
@@ -72,29 +78,6 @@ internal suspend fun probeCurrentServer() = runCatching {
     }
 }.getOrDefault(false)
 
-internal suspend fun authBranch(
-    username: String,
-    password: String,
-    wrongPasswordMessage: String,
-    rateLimitMessage: String,
-    unexpectedError: String,
-) = login(
-    username.trim(),
-    password,
-    wrongPasswordMessage,
-    rateLimitMessage,
-    unexpectedError,
-).let { result ->
-    if (
-        result is PasswordStepResult.WrongPassword &&
-        !runCatching { ApiClient.checkUsername(username.trim()).exists }.getOrDefault(true)
-    ) {
-        PasswordStepResult.AdvanceToRegister
-    } else {
-        result
-    }
-}
-
 private suspend fun fullLogin(
     username: String,
     password: String,
@@ -123,20 +106,25 @@ private suspend fun fullLogin(
     runCatching { ApiClient.refreshServerInstanceFingerprint() }
 }
 
-private suspend fun login(
+internal suspend fun authPasswordStep(
     username: String,
     password: String,
     wrongPasswordMessage: String,
     rateLimitMessage: String,
     unexpectedError: String,
 ) = try {
-    fullLogin(username, password.trim()) {
+    val derived = deriveAuthSecret(username.trim(), password.trim())
-        ApiClient.loginRequest(
+    when (val outcome = ApiClient.authPasswordStep(username.trim(), derived)) {
-            LoginRequest(username, deriveAuthSecret(username, password.trim())),
+        is ApiClient.AuthPasswordStepOutcome.LoggedIn -> {
+            fullLogin(username.trim(), password.trim()) { outcome.response }
+            PasswordStepResult.LoginSuccess
+        }
+
+        is ApiClient.AuthPasswordStepOutcome.NeedsRegister -> PasswordStepResult.NeedsRegister(
+            yandexRequired = outcome.yandexRequired,
+            yandex = outcome.yandex,
         )
     }
-
-    PasswordStepResult.LoginSuccess
 } catch (e: ClientRequestException) {
     when (e.response.status.value) {
         401 -> PasswordStepResult.WrongPassword(
@@ -158,27 +146,23 @@ internal suspend fun register(
     displayName: String,
     password: String,
     bio: String,
+    registrationProof: String?,
     unexpectedError: String,
 ) = try {
-    if (runCatching { ApiClient.checkUsername(username.trim()).exists }.getOrDefault(false)) {
+    fullLogin(username.trim(), password.trim()) {
-        RegisterResult.UsernameTaken
+        val derived = deriveAuthSecret(username.trim(), password.trim())
-    } else {
+        ApiClient.authRegisterConfirm(
-        fullLogin(username.trim(), password.trim()) {
+            RegisterConfirmRequest(
-            val derived = deriveAuthSecret(username.trim(), password.trim())
+                username = username.trim(),
-
+                display_name = displayName.trim(),
-            ApiClient.registerRequest(
+                password = derived,
-                RegisterRequest(
+                confirm_password = derived,
-                    username = username.trim(),
+                bio = bio.trim().takeIf { it.isNotEmpty() },
-                    display_name = displayName.trim(),
+                registration_proof = registrationProof,
-                    password = derived,
+            ),
-                    confirm_password = derived,
+        )
-                    bio = bio.trim().takeIf { it.isNotEmpty() },
-                ),
-            )
-        }
-
-        RegisterResult.Success
     }
+    RegisterResult.Success
 } catch (e: ClientRequestException) {
     if (e.response.status.value == 400 && isUsernameTakenError(e)) {
         RegisterResult.UsernameTaken
@@ -225,11 +209,26 @@ fun AuthScreen(
     val snackbarHostState = remember { SnackbarHostState() }
     val flowState = rememberExpressiveStepFlow(AuthFlowStep.entries.size)
 
-    var username by remember { mutableStateOf("") }
+    var username by remember { mutableStateOf(AuthRegisterDraft.username) }
-    var password by remember { mutableStateOf("") }
+    var password by remember { mutableStateOf(AuthRegisterDraft.password) }
-    var confirmPassword by remember { mutableStateOf("") }
+    var confirmPassword by remember { mutableStateOf(AuthRegisterDraft.confirmPassword) }
-    var displayName by remember { mutableStateOf("") }
+    var displayName by remember { mutableStateOf(AuthRegisterDraft.displayName) }
-    var bio by remember { mutableStateOf("") }
+    var bio by remember { mutableStateOf(AuthRegisterDraft.bio) }
+    var yandexRequired by remember { mutableStateOf(AuthRegisterDraft.yandexRequired) }
+    var yandexParams by remember { mutableStateOf(AuthRegisterDraft.yandexParams) }
+    var registrationProof by remember { mutableStateOf(AuthRegisterDraft.registrationProof) }
+
+    fun persistDraft() {
+        AuthRegisterDraft.username = username
+        AuthRegisterDraft.password = password
+        AuthRegisterDraft.confirmPassword = confirmPassword
+        AuthRegisterDraft.displayName = displayName
+        AuthRegisterDraft.bio = bio
+        AuthRegisterDraft.yandexRequired = yandexRequired
+        AuthRegisterDraft.yandexParams = yandexParams
+        AuthRegisterDraft.registrationProof = registrationProof
+        AuthRegisterDraft.page = flowState.pagerState.currentPage
+    }
 
     fun snackbar(text: String, cause: Throwable? = null) {
         scope.showLoggedSnackbar(
@@ -240,44 +239,86 @@ fun AuthScreen(
         )
     }
 
+    val wrappedAuthSuccess: () -> Unit = {
+        AuthRegisterDraft.clear()
+        onAuthSuccess()
+    }
+
+    val wrappedBackToWelcome: () -> Unit = {
+        AuthRegisterDraft.clear()
+        onBackToWelcome()
+    }
+
     val resetToUsername: () -> Unit = {
         username = ""
         password = ""
         confirmPassword = ""
         displayName = ""
         bio = ""
+        yandexRequired = false
+        yandexParams = null
+        registrationProof = null
+        AuthRegisterDraft.clear()
         flowState.resetPredictiveState()
         scope.launch {
             flowState.pagerState.animateScrollToPage(AuthFlowStep.Username.ordinal)
         }
     }
 
-    LaunchedEffect(Unit) {
+    DisposableEffect(Unit) {
-        username = ""
+        onDispose { persistDraft() }
-        password = ""
+    }
-        confirmPassword = ""
+
-        displayName = ""
+    LaunchedEffect(username, password, confirmPassword, displayName, bio, yandexRequired, yandexParams, registrationProof) {
-        bio = ""
+        persistDraft()
     }
 
     LaunchedEffect(flowState.pagerState) {
+        val restored = AuthRegisterDraft.page
+        if (restored in 1 until AuthFlowStep.entries.size &&
+            flowState.pagerState.currentPage != restored
+        ) {
+            flowState.pagerState.scrollToPage(restored)
+        }
         var settledPage = flowState.pagerState.currentPage
         snapshotFlow { flowState.pagerState.currentPage }
             .collect { page ->
+                AuthRegisterDraft.page = page
+                if (page == AuthFlowStep.YandexId.ordinal && !yandexRequired) {
+                    val target = if (page > settledPage) {
+                        AuthFlowStep.Profile.ordinal
+                    } else {
+                        AuthFlowStep.ConfirmPassword.ordinal
+                    }
+                    settledPage = target
+                    flowState.pagerState.scrollToPage(target)
+                    return@collect
+                }
                 if (page < settledPage) {
                     when (page) {
                         AuthFlowStep.Username.ordinal -> {
                             password = ""
                             confirmPassword = ""
+                            yandexRequired = false
+                            yandexParams = null
+                            registrationProof = null
                         }
 
                         AuthFlowStep.Password.ordinal -> {
                             password = ""
                             confirmPassword = ""
+                            yandexRequired = false
+                            yandexParams = null
+                            registrationProof = null
                         }
 
                         AuthFlowStep.ConfirmPassword.ordinal -> {
-                            confirmPassword = ""
+                            // Keep confirm password when returning from Yandex ID / OAuth.
+                            registrationProof = null
+                        }
+
+                        AuthFlowStep.YandexId.ordinal -> {
+                            registrationProof = null
                         }
                     }
                 }
@@ -285,6 +326,7 @@ fun AuthScreen(
             }
     }
 
+    val yandexStep = yandexParams
     ExpressiveStepFlowScaffold(
         flowState = flowState,
         pages = listOf(
@@ -300,8 +342,11 @@ fun AuthScreen(
                 username = username,
                 password = password,
                 onPasswordChange = { password = it },
-                onLoginSuccess = onAuthSuccess,
+                onLoginSuccess = wrappedAuthSuccess,
-                onRegister = {
+                onNeedsRegister = { required, params ->
+                    yandexRequired = required
+                    yandexParams = params
+                    registrationProof = null
                     flowState.pagerState.animateScrollToPage(AuthFlowStep.ConfirmPassword.ordinal)
                 },
                 onSnackbar = ::snackbar,
@@ -311,10 +356,34 @@ fun AuthScreen(
                 onConfirmPasswordChange = { confirmPassword = it },
                 password = password,
                 onContinue = {
-                    flowState.pagerState.animateScrollToPage(AuthFlowStep.Profile.ordinal)
+                    if (yandexRequired && yandexParams != null) {
+                        flowState.pagerState.animateScrollToPage(AuthFlowStep.YandexId.ordinal)
+                    } else {
+                        flowState.pagerState.animateScrollToPage(AuthFlowStep.Profile.ordinal)
+                    }
                 },
                 onSnackbar = ::snackbar,
             ),
+            if (yandexStep != null) {
+                yandexIdStepPage(
+                    yandex = yandexStep,
+                    onProof = { proof ->
+                        registrationProof = proof
+                        flowState.pagerState.animateScrollToPage(AuthFlowStep.Profile.ordinal)
+                    },
+                    onSnackbar = ::snackbar,
+                )
+            } else {
+                confirmPasswordStepPage(
+                    confirmPassword = confirmPassword,
+                    onConfirmPasswordChange = { confirmPassword = it },
+                    password = password,
+                    onContinue = {
+                        flowState.pagerState.animateScrollToPage(AuthFlowStep.Profile.ordinal)
+                    },
+                    onSnackbar = ::snackbar,
+                )
+            },
             profileStepPage(
                 username = username,
                 displayName = displayName,
@@ -322,12 +391,13 @@ fun AuthScreen(
                 bio = bio,
                 onBioChange = { bio = it },
                 password = password,
-                onRegisterSuccess = onAuthSuccess,
+                registrationProof = registrationProof,
+                onRegisterSuccess = wrappedAuthSuccess,
                 onUsernameTaken = resetToUsername,
                 onSnackbar = ::snackbar,
             ),
         ),
         snackbarHostState = snackbarHostState,
-        onBackAtFirstPage = onBackToWelcome,
+        onBackAtFirstPage = wrappedBackToWelcome,
     )
 }

app/shared/src/commonMain/kotlin/ru/fromchat/ui/auth/PasswordStep.kt

@@ -35,6 +35,7 @@ import ru.fromchat.login
 import ru.fromchat.password
 import ru.fromchat.password_length_error
 import ru.fromchat.show_password
+import ru.fromchat.api.schema.user.auth.YandexOAuthParams
 import ru.fromchat.ui.components.ActionButton
 import ru.fromchat.ui.components.ExpressiveHeroSpec
 import ru.fromchat.ui.components.ExpressiveStepLazyListIndices
@@ -53,7 +54,7 @@ internal fun passwordStepPage(
     password: String,
     onPasswordChange: (String) -> Unit,
     onLoginSuccess: () -> Unit,
-    onRegister: suspend () -> Unit,
+    onNeedsRegister: suspend (yandexRequired: Boolean, yandex: YandexOAuthParams?) -> Unit,
     onSnackbar: (String, Throwable?) -> Unit,
 ): ExpressiveStepPage {
     val scope = rememberCoroutineScope()
@@ -121,7 +122,7 @@ internal fun passwordStepPage(
 
                         try {
                             when (
-                                val result = authBranch(
+                                val result = authPasswordStep(
                                     username = username,
                                     password = password,
                                     wrongPasswordMessage = wrongPassword,
@@ -133,8 +134,8 @@ internal fun passwordStepPage(
                                     onLoginSuccess()
                                 }
 
-                                is PasswordStepResult.AdvanceToRegister -> {
+                                is PasswordStepResult.NeedsRegister -> {
-                                    onRegister()
+                                    onNeedsRegister(result.yandexRequired, result.yandex)
                                 }
 
                                 is PasswordStepResult.WrongPassword -> {

app/shared/src/commonMain/kotlin/ru/fromchat/ui/auth/UsernameStep.kt

@@ -17,12 +17,17 @@ import androidx.compose.runtime.setValue
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.focus.FocusRequester
 import androidx.compose.ui.focus.focusRequester
+import io.ktor.client.call.body
+import io.ktor.client.plugins.ClientRequestException
 import kotlinx.coroutines.launch
 import org.jetbrains.compose.resources.stringResource
 import ru.fromchat.Res
+import ru.fromchat.api.ApiClient
+import ru.fromchat.api.schema.core.ErrorResponse
 import ru.fromchat.auth_server_connect_failed
 import ru.fromchat.auth_step_username_body
 import ru.fromchat.auth_step_username_title
+import ru.fromchat.error_unexpected
 import ru.fromchat.fill_all_fields
 import ru.fromchat.settings_next
 import ru.fromchat.ui.components.ActionButton
@@ -55,6 +60,7 @@ internal fun usernameStepPage(
     val fillAll = stringResource(Res.string.fill_all_fields)
     val usernameLenError = stringResource(Res.string.username_length_error)
     val serverFail = stringResource(Res.string.auth_server_connect_failed)
+    val unexpected = stringResource(Res.string.error_unexpected)
     val nextLabel = stringResource(Res.string.settings_next)
 
     return ExpressiveStepPage(
@@ -109,7 +115,21 @@ internal fun usernameStepPage(
                             if (!probeCurrentServer()) {
                                 onSnackbar(serverFail, null)
                             } else {
-                                onContinue()
+                                try {
+                                    ApiClient.authUsernameStep(trimmed)
+                                    onContinue()
+                                } catch (e: ClientRequestException) {
+                                    val detail = if (e.response.status.value == 400) {
+                                        runCatching { e.response.body<ErrorResponse>().detail }
+                                            .getOrNull()
+                                            ?.ifBlank { null }
+                                    } else {
+                                        null
+                                    }
+                                    onSnackbar(detail ?: unexpected, e)
+                                } catch (e: Exception) {
+                                    onSnackbar(unexpected, e)
+                                }
                             }
                         } finally {
                             busy = false

app/shared/src/commonMain/kotlin/ru/fromchat/ui/auth/register/ProfileStep.kt

@@ -55,6 +55,7 @@ internal fun profileStepPage(
     bio: String,
     onBioChange: (String) -> Unit,
     password: String,
+    registrationProof: String?,
     onRegisterSuccess: () -> Unit,
     onUsernameTaken: () -> Unit,
     onSnackbar: (String, Throwable?) -> Unit,
@@ -147,6 +148,7 @@ internal fun profileStepPage(
                                     displayName = displayName.trim(),
                                     password = password,
                                     bio = bio.trim(),
+                                    registrationProof = registrationProof,
                                     unexpectedError = unexpected,
                                 )
                             ) {

app/shared/src/commonMain/kotlin/ru/fromchat/ui/auth/yandex/YandexIdStep.kt

@@ -0,0 +1,130 @@
+package ru.fromchat.ui.auth.yandex
+
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.material3.ExperimentalMaterial3ExpressiveApi
+import androidx.compose.material3.MaterialShapes
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.rememberUpdatedState
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.text.intl.Locale
+import org.jetbrains.compose.resources.stringResource
+import org.jetbrains.compose.resources.vectorResource
+import ru.fromchat.Res
+import ru.fromchat.api.schema.user.auth.YandexOAuthParams
+import ru.fromchat.auth.yandex.YANDEX_OAUTH_REDIRECT_URI
+import ru.fromchat.auth.yandex.buildYandexAuthorizeUrl
+import ru.fromchat.auth.yandex.generatePkcePair
+import ru.fromchat.auth.yandex.resolveYandexClientId
+import ru.fromchat.auth_step_yandex_body
+import ru.fromchat.auth_step_yandex_cta
+import ru.fromchat.auth_step_yandex_title
+import ru.fromchat.auth_yandex_client_mismatch
+import ru.fromchat.config.Settings
+import ru.fromchat.error_unexpected
+import ru.fromchat.ic_yandex
+import ru.fromchat.ui.LocalNavController
+import ru.fromchat.ui.components.ActionButton
+import ru.fromchat.ui.components.ExpressiveHeroSpec
+import ru.fromchat.ui.components.ExpressiveStepPage
+import ru.fromchat.ui.components.ExpressiveStepPageHeader
+import ru.fromchat.ui.components.Text
+import ru.fromchat.ui.isAppInDarkTheme
+
+@OptIn(ExperimentalMaterial3ExpressiveApi::class)
+@Composable
+internal fun yandexIdStepPage(
+    yandex: YandexOAuthParams,
+    onProof: suspend (String) -> Unit,
+    onSnackbar: (String, Throwable?) -> Unit,
+): ExpressiveStepPage {
+    val navController = LocalNavController.current
+    val colorScheme = MaterialTheme.colorScheme
+    var busy by remember { mutableStateOf(false) }
+    val languageTag = Locale.current.toLanguageTag()
+    val darkTheme = isAppInDarkTheme()
+    val onProofState = rememberUpdatedState(onProof)
+    val onSnackbarState = rememberUpdatedState(onSnackbar)
+    val yandexIcon = vectorResource(Res.drawable.ic_yandex)
+
+    val title = stringResource(Res.string.auth_step_yandex_title)
+    val body = stringResource(Res.string.auth_step_yandex_body)
+    val cta = stringResource(Res.string.auth_step_yandex_cta)
+    val clientMismatch = stringResource(Res.string.auth_yandex_client_mismatch)
+    val unexpected = stringResource(Res.string.error_unexpected)
+
+    LaunchedEffect(navController) {
+        val handle = navController.currentBackStackEntry?.savedStateHandle ?: return@LaunchedEffect
+        handle.getStateFlow<String?>(YandexOAuthNav.RESULT_PROOF, null).collect { proof ->
+            if (proof == null) return@collect
+            handle.remove<String>(YandexOAuthNav.RESULT_PROOF)
+            busy = true
+            try {
+                onProofState.value(proof)
+            } finally {
+                busy = false
+            }
+        }
+    }
+
+    LaunchedEffect(navController) {
+        val handle = navController.currentBackStackEntry?.savedStateHandle ?: return@LaunchedEffect
+        handle.getStateFlow<String?>(YandexOAuthNav.RESULT_ERROR, null).collect { message ->
+            if (message == null) return@collect
+            handle.remove<String>(YandexOAuthNav.RESULT_ERROR)
+            onSnackbarState.value(message, null)
+        }
+    }
+
+    return ExpressiveStepPage(
+        hero = ExpressiveHeroSpec(
+            icon = yandexIcon,
+            polygon = MaterialShapes.Cookie9Sided.normalized(),
+            containerColor = colorScheme.primaryContainer,
+            contentColor = colorScheme.onPrimaryContainer,
+        ),
+        content = {
+            ExpressiveStepPageHeader(title = title, body = body)
+        },
+        button = {
+            ActionButton(
+                onClick = {
+                    if (busy) return@ActionButton
+                    val serverIp = runCatching { Settings.serverConfig.serverIp }.getOrElse {
+                        onSnackbar(unexpected, it)
+                        return@ActionButton
+                    }
+                    val clientId = resolveYandexClientId(yandex.client_id, serverIp)
+                    if (clientId == null) {
+                        onSnackbar(clientMismatch, null)
+                        return@ActionButton
+                    }
+                    val pkce = generatePkcePair()
+                    YandexOAuthNav.pending = YandexOAuthNav.Session(
+                        authorizeUrl = buildYandexAuthorizeUrl(
+                            authorizeUrl = yandex.authorize_url,
+                            clientId = clientId,
+                            redirectUri = yandex.redirect_uri.ifBlank { YANDEX_OAUTH_REDIRECT_URI },
+                            scope = yandex.scope,
+                            codeChallenge = pkce.codeChallenge,
+                            languageTag = languageTag,
+                            darkTheme = darkTheme,
+                        ),
+                        codeVerifier = pkce.codeVerifier,
+                    )
+                    navController.navigate(YandexOAuthNav.ROUTE)
+                },
+                enabled = !busy,
+                loading = busy,
+                modifier = Modifier.fillMaxWidth(),
+            ) {
+                Text(cta)
+            }
+        },
+    )
+}

app/shared/src/commonMain/kotlin/ru/fromchat/ui/auth/yandex/YandexOAuthNav.kt

@@ -0,0 +1,34 @@
+package ru.fromchat.ui.auth.yandex
+
+import androidx.compose.runtime.saveable.listSaver
+import kotlin.concurrent.Volatile
+
+/**
+ * Root [androidx.navigation.NavController] route for the Yandex OAuth WebView.
+ * Session is staged in [pending] before [navigate]; PKCE verifier must not go in the route.
+ * Prefer [SessionSaver] / rememberSaveable on the OAuth screen so pause/recreate keeps PKCE.
+ */
+internal object YandexOAuthNav {
+    const val ROUTE = "yandexOAuth"
+    const val RESULT_PROOF = "yandex_registration_proof"
+    const val RESULT_ERROR = "yandex_oauth_error"
+
+    data class Session(
+        val authorizeUrl: String,
+        val codeVerifier: String,
+    )
+
+    val SessionSaver = listSaver<Session?, String>(
+        save = { session ->
+            if (session == null) emptyList()
+            else listOf(session.authorizeUrl, session.codeVerifier)
+        },
+        restore = { saved ->
+            if (saved.size < 2) null
+            else Session(authorizeUrl = saved[0], codeVerifier = saved[1])
+        },
+    )
+
+    @Volatile
+    var pending: Session? = null
+}

app/shared/src/commonMain/kotlin/ru/fromchat/ui/auth/yandex/YandexOAuthScreen.kt

@@ -0,0 +1,171 @@
+package ru.fromchat.ui.auth.yandex
+
+import androidx.compose.animation.AnimatedVisibility
+import androidx.compose.animation.fadeIn
+import androidx.compose.animation.fadeOut
+import androidx.compose.foundation.background
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.layout.safeDrawingPadding
+import androidx.compose.material.icons.Icons
+import androidx.compose.material.icons.automirrored.filled.ArrowBack
+import androidx.compose.material3.FilledIconButton
+import androidx.compose.material3.Icon
+import androidx.compose.material3.IconButtonDefaults
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.DisposableEffect
+import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.rememberCoroutineScope
+import androidx.compose.runtime.saveable.rememberSaveable
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.text.intl.Locale
+import androidx.compose.ui.unit.dp
+import io.ktor.client.call.body
+import io.ktor.client.plugins.ClientRequestException
+import kotlinx.coroutines.launch
+import org.jetbrains.compose.resources.stringResource
+import ru.fromchat.Logger
+import ru.fromchat.Res
+import ru.fromchat.api.ApiClient
+import ru.fromchat.api.schema.core.ErrorResponse
+import ru.fromchat.auth_yandex_failed
+import ru.fromchat.back
+import ru.fromchat.ui.LocalNavController
+import ru.fromchat.ui.isAppInDarkTheme
+
+private const val LOG_TAG = "YandexOAuthScreen"
+
+@Composable
+internal fun YandexOAuthScreen() {
+    val navController = LocalNavController.current
+    val scope = rememberCoroutineScope()
+    // Survive Activity recreate / process restore — in-memory [YandexOAuthNav.pending] alone does not.
+    var session by rememberSaveable(stateSaver = YandexOAuthNav.SessionSaver) {
+        mutableStateOf(YandexOAuthNav.pending)
+    }
+    val fallbackColor = MaterialTheme.colorScheme.background
+    var chromeColor by remember { mutableStateOf(fallbackColor) }
+    var busy by remember { mutableStateOf(false) }
+    var webViewCanGoBack by remember { mutableStateOf(false) }
+    val failedMessage = stringResource(Res.string.auth_yandex_failed)
+    val backLabel = stringResource(Res.string.back)
+    val darkTheme = isAppInDarkTheme()
+    val screenId = remember { (100000..999999).random().toString(16) }
+
+    DisposableEffect(screenId) {
+        Logger.i(
+            LOG_TAG,
+            "compose enter id=$screenId sessionNull=${session == null} " +
+                "pendingNull=${YandexOAuthNav.pending == null} darkTheme=$darkTheme " +
+                "route=${navController.currentBackStackEntry?.destination?.route}",
+        )
+        onDispose {
+            Logger.i(LOG_TAG, "compose dispose id=$screenId")
+        }
+    }
+
+    LaunchedEffect(session) {
+        if (session == null) {
+            Logger.w(LOG_TAG, "session null → popBackStack id=$screenId")
+            navController.popBackStack()
+        } else {
+            YandexOAuthNav.pending = session
+            Logger.d(LOG_TAG, "session kept id=$screenId urlLen=${session!!.authorizeUrl.length}")
+        }
+    }
+
+    val active = session ?: return
+
+    fun finishWithProof(proof: String) {
+        Logger.i(LOG_TAG, "finishWithProof id=$screenId")
+        YandexOAuthNav.pending = null
+        navController.previousBackStackEntry
+            ?.savedStateHandle
+            ?.set(YandexOAuthNav.RESULT_PROOF, proof)
+        navController.popBackStack()
+    }
+
+    fun finishWithError(message: String) {
+        Logger.w(LOG_TAG, "finishWithError id=$screenId message=$message")
+        YandexOAuthNav.pending = null
+        navController.previousBackStackEntry
+            ?.savedStateHandle
+            ?.set(YandexOAuthNav.RESULT_ERROR, message)
+        navController.popBackStack()
+    }
+
+    fun cancel() {
+        if (busy) return
+        Logger.i(LOG_TAG, "cancel id=$screenId")
+        YandexOAuthNav.pending = null
+        navController.popBackStack()
+    }
+
+    Box(
+        modifier = Modifier
+            .fillMaxSize()
+            .background(chromeColor),
+    ) {
+        YandexOAuthWebView(
+            authorizeUrl = active.authorizeUrl,
+            languageTag = Locale.current.toLanguageTag(),
+            darkTheme = darkTheme,
+            fallbackColor = fallbackColor,
+            onPageBackgroundColor = { chromeColor = it },
+            onHistoryBackAvailabilityChanged = { webViewCanGoBack = it },
+            onCode = { code ->
+                if (busy) return@YandexOAuthWebView
+                scope.launch {
+                    busy = true
+                    try {
+                        val proof = ApiClient.authYandexExchange(code, active.codeVerifier).registration_proof
+                        finishWithProof(proof)
+                    } catch (e: ClientRequestException) {
+                        val detail = if (e.response.status.value == 400) {
+                            runCatching { e.response.body<ErrorResponse>().detail }
+                                .getOrNull()
+                                ?.ifBlank { null }
+                        } else {
+                            null
+                        }
+                        finishWithError(detail ?: failedMessage)
+                    } catch (_: Exception) {
+                        finishWithError(failedMessage)
+                    } finally {
+                        busy = false
+                    }
+                }
+            },
+            onError = { finishWithError(it.ifBlank { failedMessage }) },
+            onCancel = { cancel() },
+        )
+
+        // Visible when back exits to our Yandex ID step (WebView has no in-page history).
+        AnimatedVisibility(
+            visible = !webViewCanGoBack && !busy,
+            enter = fadeIn(),
+            exit = fadeOut(),
+            modifier = Modifier
+                .align(Alignment.TopStart)
+                .safeDrawingPadding()
+                .padding(start = 12.dp, top = 12.dp),
+        ) {
+            FilledIconButton(
+                onClick = { cancel() },
+                colors = IconButtonDefaults.filledIconButtonColors(
+                    containerColor = MaterialTheme.colorScheme.surfaceContainerHigh.copy(alpha = 0.92f),
+                    contentColor = MaterialTheme.colorScheme.onSurface,
+                ),
+            ) {
+                Icon(Icons.AutoMirrored.Filled.ArrowBack, contentDescription = backLabel)
+            }
+        }
+    }
+}

app/shared/src/commonMain/kotlin/ru/fromchat/ui/auth/yandex/YandexOAuthWebView.kt

@@ -0,0 +1,28 @@
+package ru.fromchat.ui.auth.yandex
+
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.graphics.Color
+
+/**
+ * Platform WebView that loads [authorizeUrl] and reports the OAuth redirect.
+ * Intercepts `fromchat://oauth/yandex` and returns the authorization code.
+ *
+ * @param languageTag BCP-47 tag for Accept-Language / WebView locale.
+ * @param darkTheme Pins WebView `prefers-color-scheme` via configuration night mode.
+ * @param fallbackColor Shown until the page background can be sampled.
+ * @param onPageBackgroundColor Reported whenever a non-transparent page background is detected.
+ * @param onHistoryBackAvailabilityChanged `true` when the WebView can go back in its own history.
+ */
+@Composable
+expect fun YandexOAuthWebView(
+    authorizeUrl: String,
+    languageTag: String,
+    darkTheme: Boolean,
+    fallbackColor: Color,
+    clearCookies: Boolean = false,
+    onPageBackgroundColor: (Color) -> Unit = {},
+    onHistoryBackAvailabilityChanged: (Boolean) -> Unit = {},
+    onCode: (String) -> Unit,
+    onError: (String) -> Unit,
+    onCancel: () -> Unit,
+)

app/shared/src/commonMain/kotlin/ru/fromchat/ui/main/settings/SettingsRoutes.kt

@@ -12,6 +12,10 @@ object SettingsRoutes {
     const val SecurityPasswordFlow = "settings/security/password"
     const val Account = "settings/account"
     const val AccountDeleteFlow = "settings/account/delete"
+    /** Confirm → OAuth WebView → Done (Done pops confirm+OAuth). */
+    const val AccountYandexFlow = "settings/account/yandex"
+    const val AccountYandexOAuth = "settings/account/yandex/oauth"
+    const val AccountYandexDone = "settings/account/yandex/done"
     const val ServerConfig = "serverConfig"
     const val About = "about"
     const val Logs = "settings/logs"

app/shared/src/commonMain/kotlin/ru/fromchat/ui/main/settings/account/AccountScreen.kt

@@ -25,6 +25,7 @@ import androidx.compose.material3.TextButton
 import androidx.compose.material3.TopAppBarDefaults
 import androidx.compose.material3.rememberTopAppBarState
 import androidx.compose.runtime.Composable
+import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
@@ -37,11 +38,15 @@ import com.pr0gramm3r101.components.Category
 import com.pr0gramm3r101.components.ListItem
 import kotlinx.coroutines.launch
 import org.jetbrains.compose.resources.stringResource
+import org.jetbrains.compose.resources.vectorResource
 import ru.fromchat.Res
 import ru.fromchat.api.ApiClient
 import ru.fromchat.back
 import ru.fromchat.cancel
+import ru.fromchat.ic_yandex
 import ru.fromchat.logout
+import ru.fromchat.settings_account_change_yandex
+import ru.fromchat.settings_account_change_yandex_d
 import ru.fromchat.settings_account_delete
 import ru.fromchat.settings_account_delete_d
 import ru.fromchat.settings_account_logout_confirm_body
@@ -57,11 +62,18 @@ fun AccountScreen(
     onBack: () -> Unit,
     onLogout: () -> Unit,
     onChangePassword: () -> Unit,
+    onChangeYandexId: () -> Unit,
     onDeleteAccount: () -> Unit,
 ) {
     val scrollBehavior = TopAppBarDefaults.exitUntilCollapsedScrollBehavior(rememberTopAppBarState())
     val scope = rememberCoroutineScope()
     var showLogoutConfirm by remember { mutableStateOf(false) }
+    var yandexAvailable by remember { mutableStateOf(false) }
+    val yandexIcon = vectorResource(Res.drawable.ic_yandex)
+
+    LaunchedEffect(Unit) {
+        yandexAvailable = runCatching { ApiClient.getAccountYandex() }.isSuccess
+    }
 
     Scaffold(
         modifier = Modifier.nestedScroll(scrollBehavior.nestedScrollConnection),
@@ -103,6 +115,24 @@ fun AccountScreen(
                     }
                 )
 
+                if (yandexAvailable) {
+                    ListItem(
+                        headline = stringResource(Res.string.settings_account_change_yandex),
+                        supportingText = stringResource(Res.string.settings_account_change_yandex_d),
+                        onClick = onChangeYandexId,
+                        leadingContent = { Icon(yandexIcon, null) },
+                        divider = true,
+                        trailingContent = {
+                            Icon(
+                                imageVector = Icons.AutoMirrored.Filled.KeyboardArrowRight,
+                                contentDescription = null,
+                                modifier = Modifier.size(24.dp),
+                                tint = MaterialTheme.colorScheme.onSurfaceVariant
+                            )
+                        }
+                    )
+                }
+
                 ListItem(
                     headline = stringResource(Res.string.settings_account_delete),
                     supportingText = stringResource(Res.string.settings_account_delete_d),

app/shared/src/commonMain/kotlin/ru/fromchat/ui/main/settings/account/changeyandex/ChangeYandexConfirmScreen.kt

@@ -0,0 +1,155 @@
+package ru.fromchat.ui.main.settings.account.changeyandex
+
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.material3.ExperimentalMaterial3ExpressiveApi
+import androidx.compose.material3.MaterialShapes
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.SnackbarDuration
+import androidx.compose.material3.SnackbarHostState
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.rememberCoroutineScope
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.text.intl.Locale
+import kotlinx.coroutines.launch
+import org.jetbrains.compose.resources.stringResource
+import org.jetbrains.compose.resources.vectorResource
+import ru.fromchat.Res
+import ru.fromchat.api.ApiClient
+import ru.fromchat.api.schema.user.auth.YandexOAuthParams
+import ru.fromchat.auth.yandex.YANDEX_OAUTH_REDIRECT_URI
+import ru.fromchat.auth.yandex.buildYandexAuthorizeUrl
+import ru.fromchat.auth.yandex.generatePkcePair
+import ru.fromchat.auth.yandex.resolveYandexClientId
+import ru.fromchat.auth_yandex_client_mismatch
+import ru.fromchat.config.Settings
+import ru.fromchat.error_unexpected
+import ru.fromchat.ic_yandex
+import ru.fromchat.settings_next
+import ru.fromchat.settings_yandex_step_confirm_body
+import ru.fromchat.settings_yandex_step_confirm_cta
+import ru.fromchat.settings_yandex_step_confirm_title
+import ru.fromchat.ui.LocalNavController
+import ru.fromchat.ui.components.ActionButton
+import ru.fromchat.ui.components.ExpressiveHeroSpec
+import ru.fromchat.ui.components.ExpressiveStepFlowScaffold
+import ru.fromchat.ui.components.ExpressiveStepPage
+import ru.fromchat.ui.components.ExpressiveStepPageHeader
+import ru.fromchat.ui.components.Text
+import ru.fromchat.ui.components.rememberExpressiveStepFlow
+import ru.fromchat.ui.components.showReplacingSnackbar
+import ru.fromchat.ui.isAppInDarkTheme
+import ru.fromchat.ui.main.settings.SettingsRoutes
+
+@OptIn(ExperimentalMaterial3ExpressiveApi::class)
+@Composable
+fun ChangeYandexConfirmScreen(onBack: () -> Unit) {
+    val navController = LocalNavController.current
+    val scope = rememberCoroutineScope()
+    val snackbarHostState = remember { SnackbarHostState() }
+    val flowState = rememberExpressiveStepFlow(1)
+    var yandex by remember { mutableStateOf<YandexOAuthParams?>(null) }
+    var loadingParams by remember { mutableStateOf(true) }
+    var busy by remember { mutableStateOf(false) }
+    val darkTheme = isAppInDarkTheme()
+    val languageTag = Locale.current.toLanguageTag()
+    val yandexIcon = vectorResource(Res.drawable.ic_yandex)
+
+    val title = stringResource(Res.string.settings_yandex_step_confirm_title)
+    val body = stringResource(Res.string.settings_yandex_step_confirm_body)
+    val cta = stringResource(Res.string.settings_yandex_step_confirm_cta)
+    val next = stringResource(Res.string.settings_next)
+    val clientMismatch = stringResource(Res.string.auth_yandex_client_mismatch)
+    val unexpected = stringResource(Res.string.error_unexpected)
+
+    fun showSnack(text: String) {
+        scope.launch {
+            snackbarHostState.showReplacingSnackbar(
+                message = text,
+                withDismissAction = false,
+                duration = SnackbarDuration.Short,
+            )
+        }
+    }
+
+    LaunchedEffect(Unit) {
+        ChangeYandexDraft.clear()
+        loadingParams = true
+        try {
+            yandex = ApiClient.getAccountYandex().yandex
+        } catch (e: Exception) {
+            showSnack(e.message ?: unexpected)
+        } finally {
+            loadingParams = false
+        }
+    }
+
+    val colorScheme = MaterialTheme.colorScheme
+    ExpressiveStepFlowScaffold(
+        flowState = flowState,
+        pages = listOf(
+            ExpressiveStepPage(
+                hero = ExpressiveHeroSpec(
+                    icon = yandexIcon,
+                    polygon = MaterialShapes.Cookie9Sided.normalized(),
+                    containerColor = colorScheme.primaryContainer,
+                    contentColor = colorScheme.onPrimaryContainer,
+                ),
+                content = {
+                    ExpressiveStepPageHeader(title = title, body = body)
+                },
+                button = {
+                    ActionButton(
+                        onClick = {
+                            if (busy || loadingParams) return@ActionButton
+                            val params = yandex
+                            if (params == null) {
+                                showSnack(unexpected)
+                                return@ActionButton
+                            }
+                            busy = true
+                            scope.launch {
+                                try {
+                                    val serverIp = runCatching { Settings.serverConfig.serverIp }.getOrElse {
+                                        showSnack(it.message ?: unexpected)
+                                        return@launch
+                                    }
+                                    val clientId = resolveYandexClientId(params.client_id, serverIp)
+                                    if (clientId == null) {
+                                        showSnack(clientMismatch)
+                                        return@launch
+                                    }
+                                    val pkce = generatePkcePair()
+                                    ChangeYandexDraft.authorizeUrl = buildYandexAuthorizeUrl(
+                                        authorizeUrl = params.authorize_url,
+                                        clientId = clientId,
+                                        redirectUri = params.redirect_uri.ifBlank { YANDEX_OAUTH_REDIRECT_URI },
+                                        scope = params.scope,
+                                        codeChallenge = pkce.codeChallenge,
+                                        languageTag = languageTag,
+                                        darkTheme = darkTheme,
+                                    )
+                                    ChangeYandexDraft.codeVerifier = pkce.codeVerifier
+                                    navController.navigate(SettingsRoutes.AccountYandexOAuth)
+                                } finally {
+                                    busy = false
+                                }
+                            }
+                        },
+                        enabled = !busy && !loadingParams && yandex != null,
+                        loading = busy || loadingParams,
+                        modifier = Modifier.fillMaxWidth(),
+                    ) {
+                        Text(if (busy || loadingParams) next else cta)
+                    }
+                },
+            ),
+        ),
+        snackbarHostState = snackbarHostState,
+        onBackAtFirstPage = onBack,
+    )
+}

app/shared/src/commonMain/kotlin/ru/fromchat/ui/main/settings/account/changeyandex/ChangeYandexDoneScreen.kt

@@ -0,0 +1,62 @@
+package ru.fromchat.ui.main.settings.account.changeyandex
+
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.material.icons.Icons
+import androidx.compose.material.icons.filled.CheckCircle
+import androidx.compose.material3.ExperimentalMaterial3ExpressiveApi
+import androidx.compose.material3.MaterialShapes
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.SnackbarHostState
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.remember
+import androidx.compose.ui.Modifier
+import org.jetbrains.compose.resources.stringResource
+import ru.fromchat.Res
+import ru.fromchat.settings_done
+import ru.fromchat.settings_yandex_step_done_body
+import ru.fromchat.settings_yandex_step_done_title
+import ru.fromchat.ui.components.ActionButton
+import ru.fromchat.ui.components.ExpressiveHeroSpec
+import ru.fromchat.ui.components.ExpressiveStepFlowScaffold
+import ru.fromchat.ui.components.ExpressiveStepPage
+import ru.fromchat.ui.components.ExpressiveStepPageHeader
+import ru.fromchat.ui.components.Text
+import ru.fromchat.ui.components.rememberExpressiveStepFlow
+
+@OptIn(ExperimentalMaterial3ExpressiveApi::class)
+@Composable
+fun ChangeYandexDoneScreen(onDone: () -> Unit) {
+    val flowState = rememberExpressiveStepFlow(1)
+    val snackbarHostState = remember { SnackbarHostState() }
+    val title = stringResource(Res.string.settings_yandex_step_done_title)
+    val body = stringResource(Res.string.settings_yandex_step_done_body)
+    val done = stringResource(Res.string.settings_done)
+    val colorScheme = MaterialTheme.colorScheme
+
+    ExpressiveStepFlowScaffold(
+        flowState = flowState,
+        pages = listOf(
+            ExpressiveStepPage(
+                hero = ExpressiveHeroSpec(
+                    icon = Icons.Filled.CheckCircle,
+                    polygon = MaterialShapes.Cookie9Sided.normalized(),
+                    containerColor = colorScheme.tertiaryContainer,
+                    contentColor = colorScheme.onTertiaryContainer,
+                ),
+                content = {
+                    ExpressiveStepPageHeader(title = title, body = body)
+                },
+                button = {
+                    ActionButton(
+                        onClick = onDone,
+                        modifier = Modifier.fillMaxWidth(),
+                    ) {
+                        Text(done)
+                    }
+                },
+            ),
+        ),
+        snackbarHostState = snackbarHostState,
+        onBackAtFirstPage = onDone,
+    )
+}

app/shared/src/commonMain/kotlin/ru/fromchat/ui/main/settings/account/changeyandex/ChangeYandexDraft.kt

@@ -0,0 +1,15 @@
+package ru.fromchat.ui.main.settings.account.changeyandex
+
+/**
+ * Stages PKCE + authorize URL while the change-Yandex OAuth WebView is open
+ * (settings composition can leave the confirm screen).
+ */
+internal object ChangeYandexDraft {
+    var authorizeUrl: String? = null
+    var codeVerifier: String? = null
+
+    fun clear() {
+        authorizeUrl = null
+        codeVerifier = null
+    }
+}

app/shared/src/commonMain/kotlin/ru/fromchat/ui/main/settings/account/changeyandex/ChangeYandexOAuthScreen.kt

@@ -0,0 +1,125 @@
+package ru.fromchat.ui.main.settings.account.changeyandex
+
+import androidx.compose.animation.AnimatedVisibility
+import androidx.compose.animation.fadeIn
+import androidx.compose.animation.fadeOut
+import androidx.compose.foundation.background
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.layout.safeDrawingPadding
+import androidx.compose.material.icons.Icons
+import androidx.compose.material.icons.automirrored.filled.ArrowBack
+import androidx.compose.material3.FilledIconButton
+import androidx.compose.material3.Icon
+import androidx.compose.material3.IconButtonDefaults
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.rememberCoroutineScope
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.text.intl.Locale
+import androidx.compose.ui.unit.dp
+import kotlinx.coroutines.launch
+import org.jetbrains.compose.resources.stringResource
+import ru.fromchat.Res
+import ru.fromchat.api.ApiClient
+import ru.fromchat.back
+import ru.fromchat.ui.LocalNavController
+import ru.fromchat.ui.auth.yandex.YandexOAuthWebView
+import ru.fromchat.ui.isAppInDarkTheme
+import ru.fromchat.ui.main.settings.SettingsRoutes
+
+@Composable
+fun ChangeYandexOAuthScreen(onBack: () -> Unit) {
+    val navController = LocalNavController.current
+    val scope = rememberCoroutineScope()
+    val authorizeUrl = remember { ChangeYandexDraft.authorizeUrl }
+    val codeVerifier = remember { ChangeYandexDraft.codeVerifier }
+    val fallbackColor = MaterialTheme.colorScheme.background
+    var chromeColor by remember { mutableStateOf(fallbackColor) }
+    var busy by remember { mutableStateOf(false) }
+    var webViewCanGoBack by remember { mutableStateOf(false) }
+    val backLabel = stringResource(Res.string.back)
+
+    LaunchedEffect(authorizeUrl, codeVerifier) {
+        if (authorizeUrl.isNullOrBlank() || codeVerifier.isNullOrBlank()) {
+            onBack()
+        }
+    }
+
+    val url = authorizeUrl ?: return
+    val verifier = codeVerifier ?: return
+
+    fun finishSuccess() {
+        ChangeYandexDraft.clear()
+        navController.navigate(SettingsRoutes.AccountYandexDone) {
+            popUpTo(SettingsRoutes.AccountYandexFlow) { inclusive = true }
+        }
+    }
+
+    fun cancel() {
+        if (busy) return
+        ChangeYandexDraft.clear()
+        onBack()
+    }
+
+    Box(
+        modifier = Modifier
+            .fillMaxSize()
+            .background(chromeColor),
+    ) {
+        YandexOAuthWebView(
+            authorizeUrl = url,
+            languageTag = Locale.current.toLanguageTag(),
+            darkTheme = isAppInDarkTheme(),
+            fallbackColor = fallbackColor,
+            clearCookies = true,
+            onPageBackgroundColor = { chromeColor = it },
+            onHistoryBackAvailabilityChanged = { webViewCanGoBack = it },
+            onCode = { code ->
+                if (busy) return@YandexOAuthWebView
+                scope.launch {
+                    busy = true
+                    try {
+                        val proof = ApiClient.authYandexExchange(code, verifier).registration_proof
+                        ApiClient.changeAccountYandex(proof)
+                        finishSuccess()
+                    } catch (_: Exception) {
+                        cancel()
+                    } finally {
+                        busy = false
+                    }
+                }
+            },
+            onError = { cancel() },
+            onCancel = { cancel() },
+        )
+
+        // Same as register: show when back exits the flow (WebView has no in-page history).
+        AnimatedVisibility(
+            visible = !webViewCanGoBack && !busy,
+            enter = fadeIn(),
+            exit = fadeOut(),
+            modifier = Modifier
+                .align(Alignment.TopStart)
+                .safeDrawingPadding()
+                .padding(start = 12.dp, top = 12.dp),
+        ) {
+            FilledIconButton(
+                onClick = { cancel() },
+                colors = IconButtonDefaults.filledIconButtonColors(
+                    containerColor = MaterialTheme.colorScheme.surfaceContainerHigh.copy(alpha = 0.92f),
+                    contentColor = MaterialTheme.colorScheme.onSurface,
+                ),
+            ) {
+                Icon(Icons.AutoMirrored.Filled.ArrowBack, contentDescription = backLabel)
+            }
+        }
+    }
+}

app/shared/src/iosMain/kotlin/ru/fromchat/ui/auth/yandex/YandexOAuthWebView.ios.kt

@@ -0,0 +1,23 @@
+package ru.fromchat.ui.auth.yandex
+
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.ui.graphics.Color
+
+@Composable
+actual fun YandexOAuthWebView(
+    authorizeUrl: String,
+    languageTag: String,
+    darkTheme: Boolean,
+    fallbackColor: Color,
+    clearCookies: Boolean,
+    onPageBackgroundColor: (Color) -> Unit,
+    onHistoryBackAvailabilityChanged: (Boolean) -> Unit,
+    onCode: (String) -> Unit,
+    onError: (String) -> Unit,
+    onCancel: () -> Unit,
+) {
+    LaunchedEffect(authorizeUrl) {
+        onError("Yandex sign-in is not available on this platform yet.")
+    }
+}

gradle/libs.versions.toml

@@ -46,6 +46,7 @@ livekitAndroid = "2.25.2"
 livekitAndroidComposeComponents = "2.3.0"
 markdownRendererM3 = "0.41.0"
 bouncycastle = "1.79"
+webkit = "1.14.0"
 
 [libraries]
 androidx-core-splashscreen = { module = "androidx.core:core-splashscreen", version.ref = "coreSplashscreen" }
@@ -110,6 +111,7 @@ krypto = { module = "com.soywiz.korlibs.krypto:krypto", version.ref = "krypto" }
 cryptography-core = { module = "dev.whyoleg.cryptography:cryptography-core", version.ref = "cryptography-kotlin" }
 cryptography-provider-optimal = { module = "dev.whyoleg.cryptography:cryptography-provider-optimal", version.ref = "cryptography-kotlin" }
 androidx-work-runtime-ktx = { module = "androidx.work:work-runtime-ktx", version.ref = "androidxWork" }
+androidx-webkit = { module = "androidx.webkit:webkit", version.ref = "webkit" }
 sqldelight-runtime = { module = "app.cash.sqldelight:runtime", version.ref = "sqldelight" }
 sqldelight-coroutines-extensions = { module = "app.cash.sqldelight:coroutines-extensions", version.ref = "sqldelight" }
 sqldelight-driver-android = { module = "app.cash.sqldelight:android-driver", version.ref = "sqldelight" }