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

web/src/header.ts

@@ -2,7 +2,7 @@ import van from 'vanjs-core'
 import { now } from 'vanjs-router'
 import { GLOBAL_HAS_LOGIN, getApi } from './mixin'
 
-const { a, div, button, span } = van.tags
+const { a, div, span } = van.tags
 
 export const FFmpegAvailable = van.state<boolean | null>(null)
 
@@ -18,12 +18,6 @@ export const checkFFmpeg = async () => {
 export default () => {
     const classStr = (name: string) => van.derive(() => `text-nowrap nav-link ${now.val.split('/')[0] == name ? 'active' : ''}`)
 
-    const shutdown = async () => {
-        if (confirm('确定要关闭服务吗？')) {
-            await getApi('/shutdown')
-        }
-    }
-
     return div({ class: 'hstack gap-4' },
         div({ class: 'fs-4 fw-bold text-nowrap' }, 'Bilidown'),
         div({ class: 'nav nav-underline flex-nowrap overflow-auto' },
@@ -46,9 +40,7 @@ export default () => {
                 span({ class: () => `badge ${FFmpegAvailable.val === true ? 'bg-success' : FFmpegAvailable.val === false ? 'bg-danger' : 'bg-secondary'}` },
                     () => FFmpegAvailable.val === true ? 'FFmpeg ✓' : FFmpegAvailable.val === false ? 'FFmpeg ✗' : '检查中...'
                 )
-            ),
-            // 关闭按钮
-            button({ class: 'btn btn-outline-danger btn-sm', onclick: shutdown }, '关闭服务')
+            )
         )
     )
 }