feat: 站点媒体库与移动端底部导航，服务测试补强

- 新增媒体库：media_library/media_thumbs 服务（WebP 缩略图）、管理端 media 页面
- 移动端底部导航 MobileTabBar 替换 MobilePostBar
- 旧数据导入增强与测试、路由与登录会话/板块侧边栏测试补强
- site-doc 组件精简（移除 Breadcrumb），文档新增迁移公告说明

frontend/app/announcement/[id]/page.tsx

@@ -15,7 +15,6 @@ import {
   SiteDocPageNav,
   SiteDocAnnoNav,
   SiteDocMasthead,
-  SiteDocBreadcrumb,
 } from "@/components/site-doc";
 import { requestCanonical } from "@/lib/canonicalMeta";
 import { webPageJsonLd } from "@/lib/jsonLd";
@@ -115,14 +114,6 @@ export default async function AnnouncementPage({ params }: PageProps) {
         }
       >
         <article className="panel p-6 sm:p-8">
-          <SiteDocBreadcrumb
-            className="is-ruled"
-            items={[
-              { href: "/", label: "首页" },
-              { href: "/announcements", label: "站点公告" },
-              { label: a.title },
-            ]}
-          />
           <SiteDocMasthead
             kicker="站点公告"
             title={a.title}