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/components/site-doc/index.ts

@@ -4,6 +4,4 @@ export { default as AboutSiteHeader } from "./AboutSiteHeader";
 export { default as SiteDocPageNav } from "./SiteDocPageNav";
 export { default as SiteDocAnnoNav } from "./SiteDocAnnoNav";
 export { default as SiteDocMasthead } from "./SiteDocMasthead";
-export { default as SiteDocBreadcrumb } from "./SiteDocBreadcrumb";
-export type { Crumb } from "./SiteDocBreadcrumb";
 export type { SiteDocIdentityKind } from "./SiteDocIdentity";