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.

css/style.css

@@ -120,6 +120,30 @@ button[type="submit"]:hover {
   filter: brightness(1.1);
 }
 
+/* --- Newsletter signup --- */
+.newsletter-form {
+  max-width: 36rem;
+  display: grid;
+  gap: 1rem;
+}
+
+.newsletter-form .form-field input {
+  max-width: 24rem;
+}
+
+.newsletter-status {
+  margin: 0;
+  font-weight: 600;
+}
+
+.newsletter-status[data-kind="success"] {
+  color: #1e7e34;
+}
+
+.newsletter-status[data-kind="error"] {
+  color: #b3261e;
+}
+
 /* --- Reading list --- */
 .reading-category {
   margin-bottom: 2rem;