remove user-secrets approach & prepare a pilot deploy

client/next.config.mjs

@@ -7,7 +7,10 @@ const nextConfig = {
   reactStrictMode: true,
   turbopack: {
     root: '.'
-  }
+  },
+  // Emits .next/standalone — a self-contained server bundling only the traced runtime dependencies, so
+  // the Docker image carries no node_modules tree. Harmless for `npm run dev`/`npm run build` locally.
+  output: 'standalone'
 };
 
 export default withNextIntl(nextConfig);