Add Drupal headless stack with Next.js frontend

- Add Next.js frontend service (nextjs) with Dockerfile and source
- Update docker-compose.yml: image names, Drupal 11.3.3, nextjs service
- Add docker-compose.override.yml.disabled for dev hot-reload
- Add install-headless-modules.sh for OAuth/JSON:API module setup
- Add README.md with full setup and configuration guide
- Update nginx/Dockerfile and nginx.conf.template for cms. subdomain
- Update drupal/Dockerfile PHP-FPM build args
- Gitignore **/.vscode/ to prevent IDE workspace files from being tracked

drupal/nextjs/app/not-found.tsx

@@ -0,0 +1,19 @@
+export default function NotFound() {
+  return (
+    <div className="flex min-h-[50vh] flex-col items-center justify-center text-center">
+      <h1 className="mb-4 text-6xl font-bold text-gray-300">404</h1>
+      <h2 className="mb-2 text-2xl font-semibold text-gray-700">
+        Page Not Found
+      </h2>
+      <p className="mb-6 text-gray-500">
+        The page you are looking for does not exist.
+      </p>
+      <a
+        href="/"
+        className="rounded-lg bg-gray-900 px-6 py-3 text-sm font-medium text-white transition hover:bg-gray-800"
+      >
+        Go Home
+      </a>
+    </div>
+  )
+}