fix: enable Next.js standalone output for Docker builds

- Add output: 'standalone' to next.config.ts
- Fixes Docker build: .next/standalone now generated correctly

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

frontend/next.config.ts

@@ -10,6 +10,7 @@ const securityHeaders = [
 ];
 
 const nextConfig: NextConfig = {
+  output: "standalone",
   eslint: {
     // Do not fail production builds due to ESLint errors
     ignoreDuringBuilds: true,