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>

lib/main.dart

@@ -1,3 +1,5 @@
+import 'package:firebase_core/firebase_core.dart';
+import 'package:firebase_messaging/firebase_messaging.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter_bloc/flutter_bloc.dart';
@@ -16,6 +18,9 @@ import 'simple_bloc_observer.dart';
 void main() async {
   WidgetsFlutterBinding.ensureInitialized();
 
+  await Firebase.initializeApp();
+  FirebaseMessaging.onBackgroundMessage(firebaseMessagingBackgroundHandler);
+
   // Inicializacion de notificaciones locales
   await LocalNotifications.initializeLocalNotifications();