refactor: standardize admin page shell

frontend/src/features/design-system/DesignSystemPreview.tsx

@@ -2,6 +2,7 @@ import { AppDataTable, type AppDataTableColumn } from '@/components/app/AppDataT
 import { AppDetailLayout } from '@/components/app/AppDetailLayout'
 import { AppEmptyState } from '@/components/app/AppEmptyState'
 import { AppFilterBar } from '@/components/app/AppFilterBar'
+import { AppPage } from '@/components/app/AppPage'
 import { AppPageHeader } from '@/components/app/AppPageHeader'
 import { AppPriorityBadge } from '@/components/app/AppPriorityBadge'
 import { AppSection } from '@/components/app/AppSection'
@@ -33,8 +34,7 @@ const previewColumns: AppDataTableColumn<PreviewRow>[] = [
 
 export function DesignSystemPreview() {
   return (
-    <main className="min-h-screen bg-background p-6 text-foreground">
-      <div className="mx-auto flex max-w-6xl flex-col gap-6">
+    <AppPage maxWidth="6xl" contentClassName="gap-6">
         <AppPageHeader
           title="设计系统预览"
           actions={<Button>创建工单</Button>}
@@ -102,7 +102,6 @@ export function DesignSystemPreview() {
             <p className="text-sm text-muted-foreground">页面级布局只复用 App 组件，业务字段由页面自行传入。</p>
           </AppSection>
         </AppDetailLayout>
-      </div>
-    </main>
+    </AppPage>
   )
 }