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/app.dart

@@ -5,6 +5,8 @@ import 'package:phone_login/auth/phone_input_screen.dart';
 import 'package:phone_login/auth/sms_verification_screen.dart';
 import 'package:phone_login/home/home_screen.dart';
 import 'package:phone_login/profile/profile_screen.dart';
+import 'package:phone_login/services/auth_service.dart';
+import 'package:phone_login/theme/app_theme.dart';
 import 'package:provider/provider.dart';
 
 final _router = GoRouter(
@@ -27,8 +29,7 @@ final _router = GoRouter(
     final authState = Provider.of<AuthState>(context, listen: false);
     final bool loggedIn = authState.isLoggedIn;
     final bool loggingIn =
-        state.matchedLocation == '/login' ||
-        state.matchedLocation == '/sms_verify';
+        state.matchedLocation == '/login' || state.matchedLocation == '/sms_verify';
 
     if (!loggedIn && !loggingIn) {
       return '/login';
@@ -48,8 +49,15 @@ class App extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     return ChangeNotifierProvider(
-      create: (_) => AuthState(),
-      child: MaterialApp.router(routerConfig: _router),
+      create: (_) => AuthState(FirebaseAuthService()),
+      child: MaterialApp.router(
+        routerConfig: _router,
+        debugShowCheckedModeBanner: false,
+        title: 'Phone Login App',
+        theme: AppTheme.lightTheme,
+        darkTheme: AppTheme.darkTheme,
+        themeMode: ThemeMode.system,
+      ),
     );
   }
 }