feat: migrate prosappco from Firebase to NestJS REST API (Fase 2)

- Replace all Firebase* repositories with Api* repositories using HTTP + SharedPreferences JWT
- Remove Firebase.initializeApp() and firebase_messaging background handler from main.dart
- Update DI (app_di.dart) to inject Api* repositories instead of Firebase* ones
- Replace all Timestamp/cloud_firestore usage with ISO 8601 String dates
- Stub PhoneVerificationService (Firebase phone OTP → backend OTP when implemented)
- Add ApiService singleton with JWT management in lib/services/
- Legacy firebase_*_repository.dart files preserved for Fase 4 cleanup

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

lib/blocs/chat_bloc/chat_bloc.dart

@@ -8,10 +8,10 @@ part 'chat_event.dart';
 part 'chat_state.dart';
 
 class ChatBloc extends Bloc<ChatEvent, ChatState> {
-  final FirebaseChatRepository _chatRepository;
+  final ApiChatRepository _chatRepository;
 
   ChatBloc({
-    required FirebaseChatRepository chatRepository,
+    required ApiChatRepository chatRepository,
   })  : _chatRepository = chatRepository,
         super(ChatInitial()) {
     on<LoadChatEvent>(_onLoadChatEvent);