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>

gateway/wrangler.toml

@@ -0,0 +1,11 @@
+name = "matrix-push-gateway"
+main = "src/index.js"
+compatibility_date = "2024-11-01"
+workers_dev = true
+
+# After deploying, set these secrets:
+#   wrangler secret put FIREBASE_PROJECT_ID
+#   wrangler secret put FIREBASE_SERVICE_ACCOUNT
+#
+# Then set PUSH_GATEWAY_URL in MatrixSession.kt to:
+#   https://matrix-push-gateway.<your-subdomain>.workers.dev