feat(admin): 合并内容审核到内容管理，并固定后台侧栏

待审与内容库同一页处理；筛选切换去掉加载占位，避免页面闪动。

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

frontend/app/notifications/NotificationsClient.tsx

@@ -24,7 +24,7 @@ export default function NotificationsClient({ initialData }: { initialData: Noti
     if (n.type === "mention" && n.room_id) {
       router.push(`/chat/${n.room_id}`);
     } else if (n.type === "pending_review") {
-      router.push(n.post_id ? `/post/${n.post_id}` : "/admin/moderation");
+      router.push(n.post_id ? `/post/${n.post_id}` : "/admin/content?status=pending");
     } else {
       router.push(`/post/${n.post_id}`);
     }