Android app: native push notifications via JS bridge + background hold ...

WebView has no Push API, so the app shows notifications natively:

- NaviBridge JS interface (window.NaviAndroid): notify(), permission
  request with a 'navi-perm-result' event callback, background-mode
  start/stop and the Doze-exemption ask.
- BackgroundService: dataSync foreground service + partial wake lock —
  holds the process (and the WebView WebSocket) open while the app is
  minimized; quiet persistent notification with a "Отключить фон" action.
  Notification channels: silent background presence, high-importance
  messages.
- Manifest: POST_NOTIFICATIONS / FOREGROUND_SERVICE(_DATA_SYNC) /
  WAKE_LOCK / REQUEST_IGNORE_BATTERY_OPTIMIZATIONS + service entry;
  notification tap resumes MainActivity (singleTask) into open_url.
- webclient: nativeBridge composable; usePush gets a bridge mode that
  replaces web-push entirely (settings toggle keeps working); chat store
  raises 'Navi ответила' through the bridge on stream end when the
  window is hidden. Fixed a latent undefined-variable in web syncState
  (notificationsSupported -> notifSupported()).
- New unit tests for the bridge surface; 147 frontend tests + APK build
  pass.