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/admin/media/page.tsx

@@ -0,0 +1,49 @@
+import type { Metadata } from "next";
+import { Suspense } from "react";
+import { cookies } from "next/headers";
+import MediaLibrary from "./MediaLibrary";
+import { authCookieHeader } from "@/lib/cookies";
+import { fetchAdminMediaLibrary, type MediaLibraryItem } from "@/lib/api";
+import { getMeCached } from "@/lib/serverData";
+import { isSuperOrOwner } from "@/lib/roles";
+import Forbidden from "../Forbidden";
+import AdminPageHeader from "@/components/admin/AdminPageHeader";
+import { Images } from "lucide-react";
+
+export const metadata: Metadata = {
+  title: "媒体库",
+};
+
+export default async function AdminMediaPage() {
+  const cookie = authCookieHeader(await cookies());
+  const me = await getMeCached(cookie || undefined);
+  if (!me.user || !isSuperOrOwner(me.user.role)) {
+    return <Forbidden text="媒体库仅超级管理员及以上角色可查看。" />;
+  }
+
+  let items: MediaLibraryItem[] = [];
+  let counts: Record<string, number> = {};
+  let total = 0;
+  let initError = "";
+  try {
+    const res = await fetchAdminMediaLibrary(cookie || undefined);
+    items = res.items ?? [];
+    counts = res.counts ?? {};
+    total = res.total ?? 0;
+  } catch (e) {
+    initError = e instanceof Error ? e.message : "加载失败";
+  }
+
+  return (
+    <div>
+      <AdminPageHeader
+        icon={Images}
+        title="媒体库"
+        description={`站点内全部图片资源盘点（共 ${total} 张）。同名图片同时存在原图与 WebP 时仅展示 WebP，点开可查看原图。`}
+      />
+      <Suspense fallback={<p className="meta px-1 py-6">加载中…</p>}>
+        <MediaLibrary items={items} counts={counts} total={total} initError={initError} />
+      </Suspense>
+    </div>
+  );
+}