完善角色与消息权限：收紧板管内容处置，彻底删除仅站长，并统一管理删帖/评弹窗。

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/components/admin/AdminShell.tsx

@@ -27,6 +27,7 @@ import type { User } from "@/lib/api";
 import { apiAdminPendingCounts, apiLogout } from "@/lib/api";
 import {
   canModerateAny,
+  canAccessAdminMessages,
   isAdminOrAbove,
   isStaff,
   isSuperOrOwner,
@@ -76,7 +77,7 @@ const NAV_GROUPS: NavGroup[] = [
         href: "/admin/messages",
         label: "消息管理",
         icon: MessageCircle,
-        visible: (u) => isStaff(u.role),
+        visible: (u) => canAccessAdminMessages(u),
       },
       {
         href: "/admin/announcements",