feat: add serverless newsletter signup page

Adds newsletter.html with an email signup form that POSTs to a configured
serverless endpoint. The endpoint and API token live in
js/newsletter-config.js (token defaults to an empty deploy-time placeholder,
never hardcoded in page logic). Missing/invalid tokens and network failures
surface a fixed user-safe error; successful signups show a confirmation.
Links the page from the site navigation on all pages.

index.html

@@ -14,6 +14,7 @@
         <li><a href="index.html">Home</a></li>
         <li><a href="reading.html">Reading</a></li>
         <li><a href="contact.html">Contact</a></li>
+        <li><a href="newsletter.html">Newsletter</a></li>
       </ul>
     </nav>
   </header>