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/AdminNav.tsx

@@ -11,7 +11,6 @@ import {
   Palette,
   ShieldCheck,
   ArrowLeft,
-  ClipboardCheck,
   Coins,
   Globe,
   FolderOpen,
@@ -54,18 +53,12 @@ const NAV_GROUPS: NavGroup[] = [
   {
     title: "内容",
     items: [
-      {
-        href: "/admin/moderation",
-        label: "内容审核",
-        icon: ClipboardCheck,
-        visible: (u) => canModerateAny(u),
-        pendingBadge: true,
-      },
       {
         href: "/admin/content",
         label: "内容管理",
         icon: FolderOpen,
         visible: (u) => canModerateAny(u),
+        pendingBadge: true,
       },
       {
         href: "/admin/announcements",