Add cache-busting query strings and no-cache for HTML files

Proxy was caching old admin.js; ?v=3 query string forces a fresh fetch.
Also extend no-cache header to .html files so future deploys propagate immediately.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/index.html

@@ -4,7 +4,7 @@
   <meta charset="UTF-8">
   <meta name="viewport" content="width=device-width, initial-scale=1.0">
   <title>Homeschool</title>
-  <link rel="stylesheet" href="styles.css">
+  <link rel="stylesheet" href="styles.css?v=3">
 </head>
 <body>
 
@@ -366,6 +366,6 @@
     </div>
   </section>
 
-  <script src="app.js"></script>
+  <script src="app.js?v=3"></script>
 </body>
 </html>