fix(auth): show validation error toast on register form

apps/lib/features/auth/ui/screens/register_screen.dart

@@ -8,6 +8,7 @@ import '../../../../core/theme/design_tokens.dart';
 import '../../../../core/di/injection.dart';
 import '../../../../shared/widgets/app_button.dart';
 import '../../../../shared/widgets/banner/app_banner.dart';
+import '../../../../shared/widgets/toast/toast.dart';
 import '../../../../shared/widgets/toast/toast_type.dart';
 import '../../presentation/cubits/register_cubit.dart';
 import '../../data/auth_repository.dart';
@@ -52,6 +53,17 @@ class _RegisterViewState extends State<RegisterView> {
     cubit.passwordChanged(_passwordController.text);
 
     if (!cubit.state.isStep1Valid || cubit.state.isSending) {
+      String? errorMsg;
+      if (!cubit.state.username.isValid) {
+        errorMsg = '请输入有效的昵称（3-30个字符）';
+      } else if (!cubit.state.email.isValid) {
+        errorMsg = '请输入有效的邮箱地址';
+      } else if (!cubit.state.password.isValid) {
+        errorMsg = '密码至少需要6个字符';
+      }
+      if (errorMsg != null && mounted) {
+        Toast.show(context, errorMsg, type: ToastType.warning);
+      }
       return;
     }