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/login/page.tsx

@@ -7,6 +7,7 @@ import { MessagesSquare } from "lucide-react";
 import { apiLogin, apiMe, apiGetSettings } from "@/lib/api";
 import { hasAuthCookieHint } from "@/lib/cookies";
 import { useAllowComments } from "@/components/CommentsPolicyProvider";
+import { useAllowMessages } from "@/components/MessagesPolicyProvider";
 
 // 仅允许同源相对路径跳转，拦截 //host、/\\host 与绝对 URL 形式的开放重定向
 function safeRedirect(to: string | null): string {
@@ -42,6 +43,7 @@ export default function LoginPage() {
   const router = useRouter();
   const searchParams = useSearchParams();
   const allowComments = useAllowComments();
+  const allowMessages = useAllowMessages();
   const redirect = safeRedirect(searchParams.get("redirect"));
   const [username, setUsername] = useState("");
   const [password, setPassword] = useState("");
@@ -93,7 +95,7 @@ export default function LoginPage() {
           欢迎回来
         </h1>
         <p className="text-sm mt-2" style={{ color: "var(--ink-2)" }}>
-          登录后发帖{allowComments ? "、评论" : ""}与点赞
+          登录后发帖{allowComments ? "、评论" : ""}{allowMessages ? "、发消息" : ""}与点赞
         </p>
       </div>