refactor: implement Flutter best practices and proper architecture

- Create proper service layer with AuthService and FirebaseAuthService
- Implement UserModel for proper data representation
- Enhance AuthState with proper loading states and error handling
- Convert stateless widgets to stateful where appropriate
- Add proper form validation and user feedback mechanisms
- Implement comprehensive error handling and loading indicators
- Fix redirect logic in router for proper authentication flow
- Create theme system with light and dark themes
- Add shared components like LoadingIndicator
- Improve code organization following recommended architecture
- Add proper disposal of controllers and focus nodes
- Implement proper null safety handling

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

lib/main.dart

@@ -1,9 +1,8 @@
-import 'package:firebase_core/firebase_core.dart';
 import 'package:flutter/material.dart';
 import 'package:phone_login/app.dart';
 
 void main() async {
   WidgetsFlutterBinding.ensureInitialized();
-  await Firebase.initializeApp();
+
   runApp(const App());
 }