Refactor webhook URL configuration to use environment variable

- Updated questionnaire.js to use a local API endpoint as a fallback for the webhook URL.
- Modified server.js to retrieve the webhook URL from an environment variable.
- Added .env.example file to provide a template for environment configuration, including the optional webhook URL.
- Added an empty responses.json for local storage

public/js/questionnaire.js

@@ -1,6 +1,8 @@
 // This file handles the logic for the questionnaire, including collecting additional participant information after the quiz.
 
-const WEBHOOK_URL = "https://n8n.olehomelchenko.com/webhook/kse-research";
+// Configure webhook URL via environment variable on server side
+// Fallback to local API endpoint if no external webhook configured
+const WEBHOOK_URL = '/api/responses';
 
 document.addEventListener('DOMContentLoaded', function() {
     const questionnaireForm = document.getElementById('questionnaire-form');