feat: add Android/Capacitor platform support via overlay system

- Add Android project (Capacitor-generated, app ID com.paarrot.app)
- Add capacitor.config.json at repo root (webDir: cinny/dist)
- Add overlay/ directory with mobile-specific source patches:
  - tauri.ts: Capacitor native detection, LocalNotifications, permission APIs
  - matrix.ts / useAuthenticatedMediaUrl.ts: legacy /_matrix/media/ URL support
  - SystemNotification.tsx: Capacitor permission request flow
  - ClientNonUIFeatures.tsx: Capacitor notification routing
  - tsconfig.json: moduleResolution bundler, rootDir set
  - package-additions.json: @capacitor/* deps to merge at build time
- Add scripts/apply-overlay.mjs: applies overlay onto clean cinny submodule before build
- Update android:prepare script to run apply-overlay before build + cap sync

capacitor.config.json

@@ -0,0 +1,12 @@
+{
+  "appId": "com.paarrot.app",
+  "appName": "Paarrot",
+  "webDir": "cinny/dist",
+  "bundledWebRuntime": false,
+  "plugins": {
+    "LocalNotifications": {
+      "smallIcon": "ic_stat_paarrot",
+      "iconColor": "#FF8A00"
+    }
+  }
+}