refactor: remove shutdown button for Docker deployment

Docker 容器化部署是长期运行的服务，移除关闭服务按钮：
- 删除前端 header.ts 中的关闭按钮
- 删除 main.go 中的 /api/shutdown 接口
- 删除 router.go 中的 quit 路由和处理函数

Co-Authored-By: AI

internal/router/router.go

@@ -25,7 +25,6 @@ func API() *http.ServeMux {
 	router.HandleFunc("/getFields", getFields)
 	router.HandleFunc("/saveFields", saveFields)
 	router.HandleFunc("/logout", logout)
-	router.HandleFunc("/quit", quit)
 	router.HandleFunc("/getPopularVideos", getPopularVideos)
 	router.HandleFunc("/deleteTask", deleteTask)
 	router.HandleFunc("/cancelTask", cancelTask)
@@ -55,13 +54,6 @@ func getRedirectedLocation(w http.ResponseWriter, r *http.Request) {
 	}
 }
 
-func quit(w http.ResponseWriter, r *http.Request) {
-	util.Res{Success: true, Message: "退出成功"}.Write(w)
-	go func() {
-		os.Exit(0)
-	}()
-}
-
 func getFields(w http.ResponseWriter, r *http.Request) {
 	db := util.MustGetDB()
 	defer db.Close()