fix(mining-app): 验证收款方失败时显示提示信息

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/mining-app/lib/presentation/pages/asset/send_shares_page.dart

@@ -538,6 +538,15 @@ class _SendSharesPageState extends ConsumerState<SendSharesPage> {
         _isRecipientVerified = false;
         _recipientNickname = null;
       });
+      // 显示验证失败提示
+      if (mounted) {
+        ScaffoldMessenger.of(context).showSnackBar(
+          const SnackBar(
+            content: Text('用户不存在或无法转账'),
+            backgroundColor: _red,
+          ),
+        );
+      }
     }
   }