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>

.env

@@ -1,7 +1 @@
-API_KEY=AIzaSyCNpUV_4cMEL9QZx7NESXK7QAlRjRGwx_Y
-AUTH_DOMAIN=prosapp-5747a.firebaseapp.com
-PROJECT_ID=prosapp-5747a
-STORAGE_BUCKET=prosapp-5747a.appspot.com
-MESSAGING_SENDER_ID=245204384533
-APP_ID=1:245204384533:web:1b8f0b4c1d9ae21d9f871c
-MEASUREMENT_ID=G-EVBEQRZEDY
+API_BASE_URL=https://backend.prosapp.co/api/v1