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/notifications/page.tsx

@@ -1,7 +1,6 @@
 import { redirect } from "next/navigation";
 import Link from "next/link";
 import { cookies } from "next/headers";
-import { ChevronRight } from "lucide-react";
 import { authCookieHeader } from "@/lib/cookies";
 import { getMeCached } from "@/lib/serverData";
 import { fetchNotifications } from "@/lib/api";
@@ -32,12 +31,6 @@ export default async function NotificationsPage({ searchParams }: NotificationsP
 
   return (
     <div className="max-w-2xl mx-auto">
-      <nav className="meta -mt-4 sm:-mt-6 mb-1.5 flex items-center gap-1 text-[12.5px] leading-none">
-        <Link href="/" className="hover:text-[var(--accent)] transition-colors">首页</Link>
-        <ChevronRight size={13} />
-        <span style={{ color: "var(--accent)" }}>通知中心</span>
-      </nav>
-
       <h1 className="text-[26px] sm:text-[30px] font-extrabold tracking-tight mb-6" style={{ color: "var(--ink)" }}>
         通知中心
       </h1>