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]/page.tsx

@@ -56,7 +56,9 @@ export default async function UserPage({ params, searchParams }: UserPageProps)
 
   const cookie = authCookieHeader(await cookies());
   const me = await getMeCached(cookie || undefined);
-  const allowComments = (await getPublicSettingsCached()).allow_comments !== false;
+  const settings = await getPublicSettingsCached();
+  const allowComments = settings.allow_comments !== false;
+  const allowMessages = settings.allow_messages !== false;
 
   let data: UserProfile;
   try {
@@ -172,7 +174,7 @@ export default async function UserPage({ params, searchParams }: UserPageProps)
                 banned: u.banned,
               }}
             />
-            <StartDMButton userId={data.user.id} />
+            {allowMessages ? <StartDMButton userId={data.user.id} /> : null}
           </div>
         ) : null}