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>

pubspec.yaml

@@ -28,13 +28,9 @@ environment:
 # the latest version available on pub.dev. To see which dependencies have newer
 # versions available, run `flutter pub outdated`.
 dependencies:
-  cloud_firestore: ^5.0.2
   cupertino_icons: ^1.0.6
   email_validator: ^3.0.0
   file_picker: ^8.0.6
-  firebase_auth: ^5.1.1
-  firebase_core: ^3.1.1
-  firebase_storage: ^12.1.0
   fluro: ^2.0.5
   flutter:
     sdk: flutter