refactor(frontend): move repeated Tailwind chains into BEM classes

apps/frontend/src/pages/NotFoundPage.tsx

@@ -5,8 +5,8 @@ import { useI18n } from '@/i18n/i18n';
 export function NotFoundPage() {
   const { t } = useI18n();
   return (
-    <div className="flex min-h-screen items-center justify-center p-6">
-      <div className="max-w-md rounded-xl border border-border bg-surface p-8 text-center shadow-card">
+    <div className="app-shell items-center justify-center p-6">
+      <div className="empty-state max-w-md bg-surface p-8">
         <h1 className="font-display text-4xl">404</h1>
         <p className="mt-2 text-muted">{t('notFound.text')}</p>
         <Link to="/" className="mt-4 inline-block">