fix: resolve Dart analysis errors and format code

- Fixed unused field warning in phone_input_screen.dart
- Resolved undefined getter 'isoCode' by removing unused code
- Fixed undefined class 'User' error by adding proper Firebase import in user_model.dart
- Ran dart format to ensure consistent code style across all files
- All analyzer issues are now resolved

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

lib/services/auth_service.dart

@@ -44,4 +44,4 @@ class FirebaseAuthService implements AuthService {
   Future<void> signOut() async {
     await _firebaseAuth.signOut();
   }
-}
+}