feat: add shadcn theme switcher

frontend/src/components/app/AppTopbar.tsx

@@ -1,5 +1,6 @@
 import { Bell, ChevronRight, LogOut, Menu, UserRound } from 'lucide-react'
 
+import { AppThemeToggle } from '@/components/app/AppThemeToggle'
 import { Badge } from '@/components/ui/badge'
 import { Button } from '@/components/ui/button'
 import { Separator } from '@/components/ui/separator'
@@ -34,6 +35,7 @@ export function AppTopbar({ onMobileMenuClick }: AppTopbarProps) {
         <Button variant="ghost" size="icon" aria-label="通知">
           <Bell data-icon="inline-start" />
         </Button>
+        <AppThemeToggle />
         <Button variant="ghost" className="hidden gap-2 sm:inline-flex">
           <UserRound data-icon="inline-start" />
           <span>{user?.display_name || user?.username || '当前用户'}</span>