Add push notifications via local sync loop and FCM

Local notifications:
- NotificationHelper posts per-room notifications from the sync loop
- Notifications suppressed when room is active (ChatScreen DisposableEffect)
- Deferred for encrypted rooms until room key arrives; fires with real body
- Notification tap navigates to the room via pendingRoomId StateFlow
- POST_NOTIFICATIONS permission requested at runtime (API 33+)

FCM push (app killed):
- MatrixFirebaseMessagingService handles token refresh and data messages
- Pusher registered with homeserver pointing to Cloudflare Worker gateway
- gateway/src/index.js: Cloudflare Worker forwards Matrix pushes to FCM v1 API
  using a service-account JWT signed with Web Crypto
- last_sync_token persisted in SharedPreferences so restored sessions start with
  an incremental sync and fire notifications immediately (fixes missed-on-wake bug)
- Default FCM notification channel declared in manifest

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

flake.nix

@@ -37,6 +37,8 @@
             pkgs.jdk17
             androidSdk
             pkgs.matrix-commander
+            pkgs.nodejs_20
+            pkgs.wrangler
           ];
 
           ANDROID_HOME = androidSdkRoot;