fix: 修复创建任务时优先级转换错误

前端优先级使用中文（高/中/低），需要转换为英文（high/medium/low）
.toLowerCase() 无法转换中文字符

frontend/src/views/manager/assignment-center.vue

@@ -557,11 +557,18 @@ const handleCreateTask = async () => {
       createLoading.value = true
       
       try {
+        // 优先级中文转英文映射
+        const priorityMap: Record<string, string> = {
+          '高': 'high',
+          '中': 'medium',
+          '低': 'low'
+        }
+        
         // 构建请求数据
         const taskData = {
           title: taskForm.title,
           description: taskForm.description,
-          priority: taskForm.priority.toLowerCase(),
+          priority: priorityMap[taskForm.priority] || 'medium',
           deadline: taskForm.deadline,
           course_ids: taskForm.courses,
           user_ids: taskForm.assignType === 'all' ? [] : taskForm.members,