feat: implement FCM push notifications via firebase_messaging

- Add firebase_core + firebase_messaging to pubspec.yaml
- Initialize Firebase in main.dart before runApp
- NotificationBloc: request permission, get FCM token, register with
  backend (PATCH /users/me/fcm-token), show local notification on
  foreground messages, refresh token on rotation
- app_view.dart: call initializePushNotifications() on authenticated state

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

pubspec.yaml

@@ -10,6 +10,8 @@ environment:
 
 dependencies:
   animate_do: ^3.0.2
+  firebase_core: ^3.0.0
+  firebase_messaging: ^15.0.0
   animated_splash_screen: ^1.3.0
   chat_repository:
     path: packages/chat_repository