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/MessagesPolicyProvider.tsx

@@ -0,0 +1,52 @@
+"use client";
+
+import {
+  createContext,
+  useContext,
+  useEffect,
+  useRef,
+  useState,
+  type ReactNode,
+} from "react";
+import { useRouter } from "next/navigation";
+import { realtime, RT_SETTINGS_CHANGED, type RtSettingsData } from "@/lib/realtime";
+
+const AllowMessagesContext = createContext(true);
+
+/** 前台是否开放私聊/群聊（SSR 初值 + 后台开关热更新） */
+export function useAllowMessages(): boolean {
+  return useContext(AllowMessagesContext);
+}
+
+/**
+ * 根布局注入：关闭消息后前台立即隐藏入口；开启则刷新以拉回会话。
+ */
+export default function MessagesPolicyProvider({
+  allowMessages,
+  children,
+}: {
+  allowMessages: boolean;
+  children: ReactNode;
+}) {
+  const router = useRouter();
+  const [open, setOpen] = useState(allowMessages);
+  const openRef = useRef(open);
+  openRef.current = open;
+
+  useEffect(() => {
+    setOpen(allowMessages);
+  }, [allowMessages]);
+
+  useEffect(() => {
+    return realtime.on<RtSettingsData>(RT_SETTINGS_CHANGED, (data) => {
+      if (typeof data?.allow_messages !== "boolean") return;
+      if (openRef.current === data.allow_messages) return;
+      setOpen(data.allow_messages);
+      router.refresh();
+    });
+  }, [router]);
+
+  return (
+    <AllowMessagesContext.Provider value={open}>{children}</AllowMessagesContext.Provider>
+  );
+}