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/admin/announcements/AnnouncementAdmin.tsx

@@ -334,28 +334,7 @@ export default function AnnouncementAdmin({
         }
       />
 
-      {/* 计数条 */}
-      <div className="flex items-center gap-3 mb-4 flex-wrap">
-        <div
-          className="inline-flex items-center gap-4 rounded-lg px-3.5 py-2 text-[13px]"
-          style={{ background: "var(--panel-2)", color: "var(--ink-2)" }}
-        >
-          <span>
-            草稿{" "}
-            <strong className="tabular-nums" style={{ color: "var(--ink)" }}>
-              {draftCount}
-            </strong>
-          </span>
-          <span aria-hidden style={{ color: "var(--line)" }}>
-            |
-          </span>
-          <span>
-            已发布{" "}
-            <strong className="tabular-nums" style={{ color: "var(--ink)" }}>
-              {publishedCount}
-            </strong>
-          </span>
-        </div>
+      <div className="mb-4">
         <AdminSegmented<StatusFilter>
           ariaLabel="公告状态筛选"
           size="sm"