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/u/[id]/StartDMButton.tsx

@@ -5,12 +5,16 @@ import { useRouter } from "next/navigation";
 import { MessageSquare } from "lucide-react";
 import { apiOpenDirectChat } from "@/lib/api";
 import { toast } from "@/lib/toast";
+import { useAllowMessages } from "@/components/MessagesPolicyProvider";
 
 /** 用户主页：发起私聊 */
 export default function StartDMButton({ userId }: { userId: number }) {
   const router = useRouter();
+  const allowMessages = useAllowMessages();
   const [loading, setLoading] = useState(false);
 
+  if (!allowMessages) return null;
+
   const start = async () => {
     setLoading(true);
     try {