fix: switch to static export and nginx for reliable CSS serving

Node-based static servers (serve, http-server) were returning 404 for
CSS files. Switched to output: "export" in Next.js config and nginx
as the production static file server.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

next.config.mjs

@@ -1,4 +1,6 @@
 /** @type {import('next').NextConfig} */
-const nextConfig = {};
+const nextConfig = {
+  output: "export",
+};
 
 export default nextConfig;