feat: migrate Firebase → NestJS REST API backend

Replace all Firebase SDK (Auth, Firestore, Storage) with HTTP calls
to backend.prosapp.co/api/v1:

- New ApiService singleton (JWT token, GET/POST/PATCH/DELETE/upload)
- auth_provider: Firebase Auth → /auth/login, /auth/register, /auth/phone/*
- services_provider + calendar_services_provider → /services endpoints
- professional_provider + professionals_provider → /professional-info, /users/professionals
- profile_form_provider + professional_form_provider → /users/me, /storage/upload
- cities/professions/settings providers → /cities, /professions, /settings
- firebase_chat_repository → polling via /chats endpoints (3s interval)
- firebase_score_repository → polling via /comments endpoints (10s interval)
- professional_detail_provider → /users/:id + /professional-info/:id
- dashboard_view: Firestore.add → POST /services
- chat_view + rating_view: FirebaseAuth.uid → AuthProvider.user.id
- Models: Timestamp → String for createdAt fields
- google_fonts upgraded to ^8.1.0 (Dart 3.12 compat)
- Remove firebase_core, firebase_auth, cloud_firestore, firebase_storage from pubspec

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

lib/main.dart

@@ -1,4 +1,3 @@
-import 'package:firebase_core/firebase_core.dart';
 import 'package:flutter_dotenv/flutter_dotenv.dart';
 import 'package:flutter/material.dart';
 import 'package:intl/date_symbol_data_local.dart';
@@ -30,18 +29,6 @@ void main() async {
 
   await dotenv.load(fileName: ".env");
 
-  await Firebase.initializeApp(
-    options: FirebaseOptions(
-      apiKey: dotenv.env['API_KEY']!,
-      authDomain: dotenv.env['AUTH_DOMAIN'],
-      projectId: dotenv.env['PROJECT_ID']!,
-      storageBucket: dotenv.env['STORAGE_BUCKET'],
-      messagingSenderId: dotenv.env['MESSAGING_SENDER_ID']!,
-      appId: dotenv.env['APP_ID']!,
-      measurementId: dotenv.env['MEASUREMENT_ID'],
-    ),
-  );
-
   await LocalStorage.configurePrefs();
 
   await initializeDateFormatting('es_ES', null);