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.

newsletter.html

@@ -0,0 +1,39 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+  <meta charset="UTF-8" />
+  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+  <title>Newsletter | My Personal Blog</title>
+  <link rel="stylesheet" href="css/style.css" />
+</head>
+<body>
+  <header class="site-header">
+    <nav class="site-nav" aria-label="Main navigation">
+      <a class="brand" href="index.html">My Blog</a>
+      <ul class="nav-links">
+        <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" aria-current="page">Newsletter</a></li>
+      </ul>
+    </nav>
+  </header>
+  <main>
+    <h1>Newsletter</h1>
+    <p>Occasional updates on new posts — no spam, unsubscribe anytime. Enter your email to subscribe.</p>
+
+    <form id="newsletter-form" class="newsletter-form">
+      <div class="form-field">
+        <label for="newsletter-email">Email</label>
+        <input type="email" id="newsletter-email" name="email" required autocomplete="email" placeholder="you@example.com" />
+      </div>
+      <button type="submit">Subscribe</button>
+      <p id="newsletter-status" class="newsletter-status" role="status" aria-live="polite"></p>
+    </form>
+  </main>
+  <footer class="site-footer">
+    <p>&copy; 2026 My Personal Blog</p>
+  </footer>
+  <script type="module" src="js/newsletter.js"></script>
+</body>
+</html>