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/shared/models/user_model.dart

@@ -1,3 +1,5 @@
+import 'package:firebase_auth/firebase_auth.dart';
+
 class UserModel {
   final String uid;
   final String? displayName;
@@ -27,4 +29,4 @@ class UserModel {
   String toString() {
     return 'UserModel(uid: $uid, displayName: $displayName, phoneNumber: $phoneNumber, email: $email, photoURL: $photoURL)';
   }
-}
+}

lib/shared/widgets/loading_indicator.dart

@@ -5,8 +5,6 @@ class LoadingIndicator extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    return const Center(
-      child: CircularProgressIndicator(),
-    );
+    return const Center(child: CircularProgressIndicator());
   }
-}
+}