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/auth/auth_state.dart

@@ -1,16 +1,82 @@
 import 'package:flutter/material.dart';
+import 'package:phone_login/services/auth_service.dart';
+import 'package:phone_login/shared/models/user_model.dart';
+import 'package:firebase_auth/firebase_auth.dart' as firebase;
 
 class AuthState extends ChangeNotifier {
+  final AuthService _authService;
+
+  AuthState(this._authService) {
+    _authService.user.listen(_onUserChanged);
+  }
+
+  bool _isLoading = false;
+  bool get isLoading => _isLoading;
+
   bool _isLoggedIn = false;
   bool get isLoggedIn => _isLoggedIn;
 
-  void toggleLogin({bool? value}) {
-    _isLoggedIn = value ?? !_isLoggedIn;
+  String? _errorMessage;
+  String? get errorMessage => _errorMessage;
+
+  UserModel? _currentUser;
+  UserModel? get currentUser => _currentUser;
+
+  void _onUserChanged(firebase.User? user) {
+    if (user != null) {
+      _isLoggedIn = true;
+      _currentUser = UserModel.fromFirebaseUser(user);
+    } else {
+      _isLoggedIn = false;
+      _currentUser = null;
+    }
+    _isLoading = false;
     notifyListeners();
   }
 
-  void logout() {
-    _isLoggedIn = false;
+  Future<void> signInWithPhoneNumber(String phoneNumber) async {
+    _isLoading = true;
+    _errorMessage = null;
     notifyListeners();
+
+    try {
+      await _authService.signInWithPhoneNumber(phoneNumber);
+    } on Exception catch (e) {
+      _errorMessage = e.toString();
+    } finally {
+      _isLoading = false;
+      notifyListeners();
+    }
+  }
+
+  Future<void> verifyOTP(String verificationId, String otp) async {
+    _isLoading = true;
+    _errorMessage = null;
+    notifyListeners();
+
+    try {
+      await _authService.verifyOTP(verificationId, otp);
+    } on Exception catch (e) {
+      _errorMessage = e.toString();
+    } finally {
+      _isLoading = false;
+      notifyListeners();
+    }
+  }
+
+  Future<void> logout() async {
+    _isLoading = true;
+    notifyListeners();
+
+    try {
+      await _authService.signOut();
+      _isLoggedIn = false;
+      _currentUser = null;
+    } on Exception catch (e) {
+      _errorMessage = e.toString();
+    } finally {
+      _isLoading = false;
+      notifyListeners();
+    }
   }
 }