fix: create download directory before starting task

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/task/task.go

@@ -114,6 +114,13 @@ func (task *Task) Start() {
 		task.UpdateStatus(db, "error", fmt.Errorf("bilibili.GetSessdata: %v", err))
 		return
 	}
+
+	// 确保下载目录存在
+	if err := os.MkdirAll(task.Folder, os.ModePerm); err != nil {
+		task.UpdateStatus(db, "error", fmt.Errorf("创建下载目录失败: %v", err))
+		return
+	}
+
 	client := &bilibili.BiliClient{SESSDATA: sessdata}
 
 	GlobalDownloadSem.Acquire()