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/ui/views/chat_view.dart

@@ -1,5 +1,5 @@
-import 'package:firebase_auth/firebase_auth.dart';
 import 'package:flutter/material.dart';
+import 'package:prosapp_web_app/providers/auth_provider.dart';
 import 'package:flutter_rating_bar/flutter_rating_bar.dart';
 import 'package:intl/intl.dart';
 import 'package:prosapp_web_app/models/message_entity.dart';
@@ -131,13 +131,13 @@ class ChatView extends StatelessWidget {
                     padding: const EdgeInsets.only(top: 10),
                     reverse: true,
                     child: Column(
-                      children: _messagesList(chat.messages),
+                      children: _messagesList(chat.messages, Provider.of<AuthProvider>(context, listen: false).user!.id),
                     ),
                   ),
                 ),
                 _MessageInput(
                     serviceId: serviceId,
-                    userId: FirebaseAuth.instance.currentUser!.uid),
+                    userId: Provider.of<AuthProvider>(context, listen: false).user!.id),
               ],
             ),
           ),
@@ -163,10 +163,10 @@ class ChatView extends StatelessWidget {
     return resultado;
   }
 
-  List<Widget> _messagesList(List<MessageEntity> messages) {
+  List<Widget> _messagesList(List<MessageEntity> messages, String currentUserId) {
     return messages
         .map(
-          (e) => e.ownerId != FirebaseAuth.instance.currentUser!.uid
+          (e) => e.ownerId != currentUserId
               ? ListTile(
                   title: Column(
                     mainAxisAlignment: MainAxisAlignment.start,