fix: preserve /api/ prefix in Next.js rewrite to backend

The rewrite was stripping /api/ from the path, but the Go backend
expects routes at /api/tenants, /api/dashboard etc.

frontend/next.config.ts

@@ -5,7 +5,7 @@ const nextConfig: NextConfig = {
   rewrites: async () => [
     {
       source: "/api/:path*",
-      destination: `${process.env.API_URL || "http://localhost:8080"}/:path*`,
+      destination: `${process.env.API_URL || "http://localhost:8080"}/api/:path*`,
     },
   ],
 };