fix: 添加UserService.isAdminUser方法以修复编译错误

backend/src/main/java/com/designhub/service/UserService.java

@@ -1302,4 +1302,12 @@ public class UserService {
         
         return result;
     }
+
+    /**
+     * 检查用户是否是管理员
+     */
+    public boolean isAdminUser(Long userId) {
+        User user = userMapper.selectById(userId);
+        return user != null && user.getIsAdmin() != null && user.getIsAdmin() == 1;
+    }
 }