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/shared/models/user_model.dart

@@ -0,0 +1,30 @@
+class UserModel {
+  final String uid;
+  final String? displayName;
+  final String? phoneNumber;
+  final String? email;
+  final String? photoURL;
+
+  UserModel({
+    required this.uid,
+    this.displayName,
+    this.phoneNumber,
+    this.email,
+    this.photoURL,
+  });
+
+  factory UserModel.fromFirebaseUser(User user) {
+    return UserModel(
+      uid: user.uid,
+      displayName: user.displayName,
+      phoneNumber: user.phoneNumber,
+      email: user.email,
+      photoURL: user.photoURL,
+    );
+  }
+
+  @override
+  String toString() {
+    return 'UserModel(uid: $uid, displayName: $displayName, phoneNumber: $phoneNumber, email: $email, photoURL: $photoURL)';
+  }
+}