Add platform detection, fix styles

frontend/src/core/init.ts

@@ -7,6 +7,18 @@
 
 import { PRODUCT_NAME } from "./config";
 import { enableMapSet } from "immer";
+import type { Platform } from "../../electron.d";
+
+function detectPlatform(): Platform {
+    const userAgent = navigator.userAgent.toLowerCase();
+    if (userAgent.includes("win")) return "win32";
+    if (userAgent.includes("mac")) return "darwin";
+    return "linux";
+}
 
 document.title = PRODUCT_NAME;
-enableMapSet();
+enableMapSet();
+
+// Add platform class to body
+const platform = detectPlatform();
+document.body.classList.add(`platform-${platform}`);