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]/(auth)/layout.tsx

@@ -0,0 +1,7 @@
+export default function AuthLayout({ children }: { children: React.ReactNode }) {
+  return (
+    <main id="main-content" role="main" className="flex min-h-screen items-center justify-center bg-muted/40 px-4 py-12">
+      <div className="w-full max-w-md">{children}</div>
+    </main>
+  );
+}