Add basePath for proper subpath deployment with nginx

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

next.config.ts

@@ -1,7 +1,7 @@
 import type { NextConfig } from "next";
 
 const nextConfig: NextConfig = {
-  /* config options here */
+  basePath: '/leopost',
 };
 
 export default nextConfig;