fix(frontend): preserve /api prefix in Next.js rewrite

The rewrite was incorrectly configured:
- Before: /api/:path* -> http://backend:8000/:path* (strips /api)
- After: /api/:path* -> http://backend:8000/api/:path* (preserves /api)

This was causing requests to /api/v1/agent-types to be sent to
http://backend:8000/v1/agent-types instead of the correct path.

frontend/next.config.ts

@@ -79,7 +79,7 @@ const nextConfig: NextConfig = {
     return [
       {
         source: '/api/:path*',
-        destination: 'http://backend:8000/:path*',
+        destination: 'http://backend:8000/api/:path*',
       },
     ];
   },