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/components/ui/language-switcher.tsx

@@ -0,0 +1,35 @@
+'use client';
+
+import { useLocale, useTranslations } from 'next-intl';
+import type { Locale } from '@/i18n/config';
+import { usePathname, useRouter } from '@/i18n/navigation';
+
+const localeLabels: Record<Locale, string> = {
+  vi: '🇻🇳 VI',
+  en: '🇬🇧 EN',
+};
+
+export function LanguageSwitcher() {
+  const locale = useLocale() as Locale;
+  const router = useRouter();
+  const pathname = usePathname();
+  const t = useTranslations('language');
+
+  const switchLocale = (newLocale: Locale) => {
+    router.replace(pathname, { locale: newLocale });
+  };
+
+  const nextLocale: Locale = locale === 'vi' ? 'en' : 'vi';
+
+  return (
+    <button
+      type="button"
+      onClick={() => switchLocale(nextLocale)}
+      className="inline-flex h-9 items-center gap-1.5 rounded-md px-2.5 text-sm font-medium text-muted-foreground 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"
+      aria-label={`${t('label')}: ${t(locale)} → ${t(nextLocale)}`}
+    >
+      <span aria-hidden="true">{localeLabels[nextLocale]}</span>
+      <span className="sr-only">{t(nextLocale)}</span>
+    </button>
+  );
+}