chore: Add output: 'standalone' to Next.js config for Docker compatibility

- Ensures Next.js 16 generates minimal standalone output
- Required for the Dockerfile's production stage to work correctly

frontend/next.config.js

@@ -1,6 +1,7 @@
 /** @type {import('next').NextConfig} */
 const nextConfig = {
   reactStrictMode: true,
+  output: 'standalone',
   env: {
     NEXT_PUBLIC_API_URL: process.env.NEXT_PUBLIC_API_URL || 'http://localhost:3001'
   }