fix(apps): consolidate FormzInput validators and fix login screen

- Move FormzInput validators to core/form_inputs/form_inputs.dart
- Fix login_screen.dart syntax error (missing 'class' keyword)
- Remove unused _isLoading field
- Fix unnecessary const keywords
- Update login_cubit and register_cubit imports
- Remove duplicate FormzInput definitions from register_cubit
- Add Toast and Banner UI feedback system
- Remove legacy login/register screens (login_code, login_email, login_password, register_step2)
- Remove unused warning_banner widget
- Update tests for new error messages and DI setup

apps/test/widget_test.dart

@@ -4,9 +4,13 @@ import 'package:mocktail/mocktail.dart';
 import 'package:social_app/main.dart';
 import 'package:social_app/features/auth/presentation/bloc/auth_bloc.dart';
 import 'package:social_app/features/auth/presentation/bloc/auth_state.dart';
+import 'package:social_app/features/auth/data/auth_repository.dart';
+import 'package:social_app/core/di/injection.dart';
 
 class MockAuthBloc extends Mock implements AuthBloc {}
 
+class MockAuthRepository extends Mock implements AuthRepository {}
+
 class FakeAuthState extends Fake implements AuthState {}
 
 void main() {
@@ -14,6 +18,13 @@ void main() {
     registerFallbackValue(FakeAuthState());
   });
 
+  setUp(() async {
+    if (sl.isRegistered<AuthRepository>()) {
+      await sl.reset();
+    }
+    sl.registerSingleton<AuthRepository>(MockAuthRepository());
+  });
+
   testWidgets('Login screen loads correctly', (WidgetTester tester) async {
     final mockAuthBloc = MockAuthBloc();
     when(() => mockAuthBloc.state).thenReturn(AuthInitial());
@@ -23,7 +34,7 @@ void main() {
 
     await tester.pumpWidget(LinksyApp(authBloc: mockAuthBloc));
     expect(find.text('linksy'), findsOneWidget);
-    expect(find.text('继续'), findsOneWidget);
+    expect(find.text('登录'), findsOneWidget);
     expect(find.text('还没有账号？去注册'), findsOneWidget);
   });