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/chat/ChatShell.tsx

@@ -114,12 +114,14 @@ export default function ChatShell({
   const longPressTimer = useRef<ReturnType<typeof setTimeout> | null>(null);
   /** 本人刚发出的消息 ID：拉列表时若水位尚未跟上，避免把这条算成未读 */
   const lastSentIdByRoom = useRef<Map<number, number>>(new Map());
+  /** 搜索重开私聊：router.push 完成前 activeRoomId 仍为 null，列表刷新需据此抑制目标会话未读 */
+  const enteringRoomIdRef = useRef<number | null>(null);
 
   /** 拉列表/SSR 回填时，当前打开的会话未读一律置 0（已读请求可能尚未落库） */
   const applyRooms = useCallback(
     (list: ChatRoomView[]) => {
       const next = (list || []).map((r) => {
-        if (activeRoomId && r.id === activeRoomId) {
+        if ((activeRoomId && r.id === activeRoomId) || r.id === enteringRoomIdRef.current) {
           return { ...r, unread_count: 0 };
         }
         const sentId = lastSentIdByRoom.current.get(r.id);
@@ -302,6 +304,9 @@ export default function ChatShell({
   // 进房即清角标；MarkRead 成功后再清一次（防 refresh 抢在已读落库前写回旧未读）
   useEffect(() => {
     if (!activeRoomId) return;
+    if (enteringRoomIdRef.current === activeRoomId) {
+      enteringRoomIdRef.current = null;
+    }
     const clearActiveUnread = () => {
       setRooms((prev) =>
         prev.map((r) => (r.id === activeRoomId ? { ...r, unread_count: 0 } : r))
@@ -346,6 +351,7 @@ export default function ChatShell({
   const openDM = async (uid: number) => {
     try {
       const { room } = await apiOpenDirectChat(uid);
+      enteringRoomIdRef.current = room.id;
       setQuery("");
       setUserHits([]);
       router.push(`/chat/${room.id}`);
@@ -468,7 +474,7 @@ export default function ChatShell({
           )}
         </div>
 
-        <div className="flex-1 overflow-y-auto min-h-0">
+        <div className="j13-chat-scroll flex-1 overflow-y-auto min-h-0">
           {query.trim() && (
             <div className="px-2 pb-2">
               <p