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/admin/layout.tsx

@@ -42,10 +42,12 @@ export default async function AdminLayout({ children }: { children: React.ReactN
     <div>
       <AdminNavMobile user={me.user} />
       <div className="flex items-start gap-6 xl:gap-7">
-        <aside className="hidden lg:block w-60 xl:w-64 shrink-0">
-          <div className="sticky" style={{ top: 84 }}>
-            <AdminSidebar user={me.user} />
-          </div>
+        {/* sticky 必须钉在 aside 自身：内层与侧栏同高时等于没钉住，会跟着主栏一起滚 */}
+        <aside
+          className="hidden lg:block w-60 xl:w-64 shrink-0 sticky self-start z-20 max-h-[calc(100vh-6.5rem)] overflow-y-auto"
+          style={{ top: 84 }}
+        >
+          <AdminSidebar user={me.user} />
         </aside>
         <section className="flex-1 min-w-0">{children}</section>
       </div>