fix: use standard Next.js directory structure for Vercel

frontend/next.config.js

@@ -1,11 +1,13 @@
 /** @type {import('next').NextConfig} */
+const apiBase = process.env.NEXT_PUBLIC_API_URL || "http://localhost:8000/api/v1";
+
 const nextConfig = {
   reactStrictMode: true,
   async rewrites() {
     return [
       {
         source: "/api/v1/:path*",
-        destination: "http://localhost:8000/api/v1/:path*",
+        destination: `${apiBase}/:path*`,
       },
     ];
   },