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/app/chat/page.tsx

@@ -2,9 +2,10 @@ import { redirect } from "next/navigation";
 import { cookies } from "next/headers";
 import type { Metadata } from "next";
 import { authCookieHeader } from "@/lib/cookies";
-import { getMeCached } from "@/lib/serverData";
+import { getMeCached, getPublicSettingsCached } from "@/lib/serverData";
 import { fetchChatRooms } from "@/lib/api";
 import ChatShell from "./ChatShell";
+import MessagesUnsupported from "@/components/MessagesUnsupported";
 
 export const metadata: Metadata = {
   title: "消息",
@@ -12,6 +13,9 @@ export const metadata: Metadata = {
 };
 
 export default async function ChatListPage() {
+  if ((await getPublicSettingsCached()).allow_messages === false) {
+    return <MessagesUnsupported />;
+  }
   const cookie = authCookieHeader(await cookies());
   const me = await getMeCached(cookie || undefined);
   if (!me.user) {