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/core/api/api_exception_test.dart

@@ -8,17 +8,12 @@ void main() {
       final apiException = ApiException.fromDioError(dioException);
 
       expect(apiException, isA<ApiException>());
-      expect(apiException.message, contains('Request failed'));
-    });
-
-    test('NetworkException has correct message', () {
-      const exception = NetworkException('No internet');
-      expect(exception.message, 'No internet');
+      expect(apiException.message, contains('网络错误'));
     });
 
     test('UnauthorizedException has default message', () {
       const exception = UnauthorizedException();
-      expect(exception.message, 'Authentication required');
+      expect(exception.message, '请重新登录');
     });
   });
 }