feat(web): add i18n locale routes and language switcher component

Add locale-prefixed routes for admin, auth, dashboard, and public pages.
Add error, loading, and not-found pages for locale context. Add language
switcher UI component for Vietnamese/English toggle.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/web/app/[locale]/not-found.tsx

@@ -0,0 +1,34 @@
+import { useTranslations } from 'next-intl';
+import { Link } from '@/i18n/navigation';
+
+export default function NotFound() {
+  const t = useTranslations();
+
+  return (
+    <div className="flex min-h-screen flex-col items-center justify-center bg-background px-4">
+      <div className="mx-auto max-w-md text-center">
+        <div className="text-8xl font-bold text-primary/20" aria-hidden="true">404</div>
+        <h1 className="mt-4 text-2xl font-bold tracking-tight">
+          {t('notFound.title')}
+        </h1>
+        <p className="mt-2 text-muted-foreground">
+          {t('notFound.description')}
+        </p>
+        <div className="mt-8 flex justify-center gap-3">
+          <Link
+            href="/"
+            className="inline-flex h-10 items-center justify-center rounded-md bg-primary px-6 text-sm font-medium text-primary-foreground shadow transition-colors hover:bg-primary/90 focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2"
+          >
+            {t('common.goHome')}
+          </Link>
+          <Link
+            href="/search"
+            className="inline-flex h-10 items-center justify-center rounded-md border border-input bg-background px-6 text-sm font-medium shadow-sm transition-colors hover:bg-accent hover:text-accent-foreground focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2"
+          >
+            {t('common.search')}
+          </Link>
+        </div>
+      </div>
+    </div>
+  );
+}