fix: properly resolve relative file

Related: socketio/socket.io-website@6a62d7d

index.js

@@ -1,5 +1,7 @@
 import express from 'express';
 import { createServer } from 'node:http';
+import { fileURLToPath } from 'node:url';
+import { dirname, join } from 'node:path';
 import { Server } from 'socket.io';
 import sqlite3 from 'sqlite3';
 import { open } from 'sqlite';
@@ -37,8 +39,10 @@ if (cluster.isPrimary) {
     adapter: createAdapter()
   });
 
+  const __dirname = dirname(fileURLToPath(import.meta.url));
+
   app.get('/', (req, res) => {
-    res.sendFile(new URL('./index.html', import.meta.url).pathname);
+    res.sendFile(join(__dirname, 'index.html'));
   });
 
   io.on('connection', async (socket) => {