Update site URLs to use the 'www' subdomain across the application

- Changed the site URL in index.html, router, and server routes from 'https://ypchat.net' to 'https://www.ypchat.net' for consistency and improved SEO.
- Updated related meta tags and constants to reflect the new URL structure.

These changes ensure a unified domain reference throughout the application.

server/routes-seo.js

@@ -2,7 +2,7 @@ import { readFileSync, existsSync } from 'fs';
 import { join, resolve } from 'path';
 import { loadFeedback } from './feedback-store.js';
 
-const SITE_URL = 'https://ypchat.net';
+const SITE_URL = 'https://www.ypchat.net';
 const DEFAULT_IMAGE = `${SITE_URL}/static/favicon.png`;
 
 const seoData = {