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/SiteChrome.tsx

@@ -5,6 +5,7 @@ import { usePathname } from "next/navigation";
 import Header from "@/components/Header";
 import Footer from "@/components/Footer";
 import BackToTop from "@/components/BackToTop";
+import MobileTabBar from "@/components/MobileTabBar";
 import type { User } from "@/lib/api";
 import { useSiteBrand } from "@/components/SiteBrandProvider";
 
@@ -23,10 +24,9 @@ function pathIsComposeStudio(pathname: string | null) {
   return pathname === "/compose" || /^\/post\/[^/]+\/edit\/?$/.test(pathname);
 }
 
-/** 帖子详情（非 edit）移动端有底栏，回顶需抬高 */
-function pathIsPostDetail(pathname: string | null) {
-  if (!pathname) return false;
-  return /^\/post\/[^/]+$/.test(pathname);
+/** 底部公共菜单避让：聊天 / 发帖工作台锁视口，不渲染底栏 */
+function pathHasNoTabBar(pathname: string | null) {
+  return pathIsChat(pathname) || pathIsComposeStudio(pathname);
 }
 
 /**
@@ -85,6 +85,7 @@ export default function SiteChrome({
 
   // 聊天 / 发帖工作台锁视口，窗口回顶无意义
   const showBackToTop = !pathIsChat(pathname) && !pathIsComposeStudio(pathname);
+  const showTabBar = showBackToTop;
 
   return (
     <>
@@ -95,7 +96,6 @@ export default function SiteChrome({
         initialTheme={theme}
         siteName={brand.site_name}
         wordmark={brand.site_wordmark}
-        slogan={brand.site_slogan}
         logoLight={brand.logo_light_url}
         logoDark={brand.logo_dark_url}
         brandMark={brand.brand_mark}
@@ -105,14 +105,15 @@ export default function SiteChrome({
       <main
         id="main"
         tabIndex={-1}
-        className="flex-1 w-full max-w-[1440px] mx-auto px-4 sm:px-6 py-7 sm:py-[var(--main-pad-y)] outline-none"
+        className="flex-1 w-full max-w-[1440px] mx-auto px-4 sm:px-6 pt-4 sm:pt-[var(--main-pad-y)] pb-[calc(4.75rem+env(safe-area-inset-bottom))] lg:pb-[var(--main-pad-y)] outline-none"
       >
         {children}
       </main>
       <Footer siteName={brand.site_name} links={brand.footer_links} />
-      {showBackToTop ? (
-        <BackToTop liftForMobileBar={pathIsPostDetail(pathname)} />
+      {showTabBar ? (
+        <MobileTabBar user={user} initialChatUnread={chatUnread} />
       ) : null}
+      {showBackToTop ? <BackToTop /> : null}
     </>
   );
 }