feat: 后台增加开放消息开关

关闭后前台隐藏私聊/群聊入口并由后端拦截；同步去掉公告页重复计数胶囊。

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/components/Header.tsx

@@ -29,6 +29,7 @@ import { onDocumentVisible } from "@/lib/visibilityReconcile";
 import NotificationBell from "./NotificationBell";
 import ThemeToggle from "./ThemeToggle";
 import Avatar from "./Avatar";
+import { useAllowMessages } from "./MessagesPolicyProvider";
 
 export default function Header({
   initialUser,
@@ -47,6 +48,7 @@ export default function Header({
 }) {
   const router = useRouter();
   const pathname = usePathname();
+  const allowMessages = useAllowMessages();
   const [menuOpen, setMenuOpen] = useState(false);
   // 登录态以 SSR 直出为初始值，F5 无切换闪动；router.refresh() 后随 props 校正
   const [user, setUser] = useState<User | null>(initialUser);
@@ -105,7 +107,7 @@ export default function Header({
 
   // 实时未读：chat:unread 推送给所有成员（含未打开会话页）；在当前会话页则忽略
   useEffect(() => {
-    if (!user) return;
+    if (!user || !allowMessages) return;
     const off = realtime.on<RtChatUnreadData>(RT_CHAT_UNREAD, (data) => {
       if (!data || typeof data.room_id !== "number") return;
       if (pathname === `/chat/${data.room_id}`) return;
@@ -113,11 +115,11 @@ export default function Header({
       setChatUnread((c) => Math.max(0, c + delta));
     });
     return off;
-  }, [user, pathname]);
+  }, [user, pathname, allowMessages]);
 
   // 会话内已读 / 回前台：合并对账（短时间多次 markRead 只打一次 summary）
   useEffect(() => {
-    if (!user) return;
+    if (!user || !allowMessages) return;
     let debounceTimer: ReturnType<typeof setTimeout> | null = null;
     const pullSummary = () => {
       apiChatUnreadSummary()
@@ -135,7 +137,7 @@ export default function Header({
       window.removeEventListener("j13-chat-unread-sync", onSync);
       offVisible();
     };
-  }, [user]);
+  }, [user, allowMessages]);
 
   const handleSearch = (e: React.FormEvent) => {
     e.preventDefault();
@@ -271,7 +273,7 @@ export default function Header({
             className={`${iconBtn} hidden lg:inline-flex`}
             style={iconBtnStyle}
           />
-          {user && (
+          {user && allowMessages && (
             <Link
               href="/chat"
               prefetch={false}
@@ -359,7 +361,7 @@ export default function Header({
           <div className="flex flex-col">
             <MobileLink href="/" active={pathname === "/"}>首页</MobileLink>
             {user && <MobileLink href="/compose">发帖</MobileLink>}
-            {user && (
+            {user && allowMessages && (
               <MobileLink href="/chat" active={pathname?.startsWith("/chat") ?? false}>
                 <span className="inline-flex items-center gap-2">
                   <MessageSquare size={15} /> 消息