feat: add task cancellation support

- Add CancelTask function to cancel running/waiting tasks
- Add /api/cancelTask endpoint
- Add cancel button in task list for running/waiting tasks
- Check cancellation status during download and merge operations

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

internal/router/task.go

@@ -170,3 +170,17 @@ func deleteTask(w http.ResponseWriter, r *http.Request) {
 	}
 	util.Res{Success: true, Message: "删除成功"}.Write(w)
 }
+
+func cancelTask(w http.ResponseWriter, r *http.Request) {
+	taskIDStr := r.FormValue("id")
+	taskID, err := strconv.ParseInt(taskIDStr, 10, 64)
+	if err != nil {
+		util.Res{Success: false, Message: "参数错误"}.Write(w)
+		return
+	}
+	if task.CancelTask(taskID) {
+		util.Res{Success: true, Message: "任务已取消"}.Write(w)
+	} else {
+		util.Res{Success: false, Message: "任务不存在或已完成"}.Write(w)
+	}
+}