Add Kotlin plugin system (SDK, host, hooks, settings UI)

Implement exteraGram-style plugin architecture for Android and desktop:
- plugins/sdk: BasePlugin API, hook types, settings DSL
- plugins/host: PluginEngine, typed send hooks, FeatureGate, HookRegistry
- JVM ByteBuddy and Android Pine hook backends
- DevServer on port 42690 (JVM)
- Plugins settings screens and PluginUiHost bulletin overlay
- hello_world sample plugin packaged as .fcplugin
- ProGuard keepnames for ru.fromchat.** with shrinking enabled

Co-authored-by: denis0001-dev <denis0001.dev@ya.ru>

plugins/sample-hello/build.gradle.kts

@@ -0,0 +1,62 @@
+plugins {
+    kotlin("jvm")
+}
+
+dependencies {
+    implementation(project(":plugins:sdk"))
+}
+
+tasks.register("packageFcPlugin") {
+    group = "plugins"
+    description = "Build hello_world.fcplugin with desktop JAR"
+    dependsOn(tasks.named("compileKotlin"))
+    doLast {
+        val buildDir = layout.buildDirectory.get().asFile
+        val staging = buildDir.resolve("fcplugin-staging")
+        staging.deleteRecursively()
+        staging.mkdirs()
+        staging.resolve("desktop").mkdirs()
+        staging.resolve("android").mkdirs()
+
+        val jvmClasses = buildDir.resolve("classes/kotlin/main")
+        val jarFile = staging.resolve("desktop/plugin.jar")
+        val jarProcess = ProcessBuilder(
+            "jar",
+            "cf",
+            jarFile.absolutePath,
+            "-C",
+            jvmClasses.absolutePath,
+            ".",
+        ).inheritIO().start()
+        check(jarProcess.waitFor() == 0) { "jar failed" }
+
+        staging.resolve("manifest.json").writeText(
+            """
+            {
+              "id": "hello_world",
+              "name": "Hello World",
+              "description": "Rewrites .hello commands into a friendly greeting",
+              "author": "FromChat",
+              "version": "1.0.0",
+              "app_version": ">=1.0.0",
+              "sdk_version": ">=1.0.0",
+              "entry_class": "ru.fromchat.plugins.sample.hello.HelloWorldPlugin",
+              "artifacts": {
+                "android": "android/plugin.dex",
+                "desktop": "desktop/plugin.jar"
+              }
+            }
+            """.trimIndent(),
+        )
+
+        val archive = buildDir.resolve("distributions/hello_world.fcplugin")
+        archive.parentFile.mkdirs()
+        if (archive.exists()) archive.delete()
+        val zipProcess = ProcessBuilder("zip", "-r", archive.absolutePath, ".")
+            .directory(staging)
+            .inheritIO()
+            .start()
+        check(zipProcess.waitFor() == 0) { "zip failed" }
+        println("Created ${archive.absolutePath}")
+    }
+}