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

android/variables.gradle

@@ -0,0 +1,16 @@
+ext {
+    minSdkVersion = 24
+    compileSdkVersion = 36
+    targetSdkVersion = 36
+    androidxActivityVersion = '1.11.0'
+    androidxAppCompatVersion = '1.7.1'
+    androidxCoordinatorLayoutVersion = '1.3.0'
+    androidxCoreVersion = '1.17.0'
+    androidxFragmentVersion = '1.8.9'
+    coreSplashScreenVersion = '1.2.0'
+    androidxWebkitVersion = '1.14.0'
+    junitVersion = '4.13.2'
+    androidxJunitVersion = '1.3.0'
+    androidxEspressoCoreVersion = '3.7.0'
+    cordovaAndroidVersion = '14.0.1'
+}