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.

js/newsletter-config.js

@@ -0,0 +1,17 @@
+/**
+ * Newsletter signup configuration.
+ *
+ * The blog is a static site with no build step, so the serverless endpoint
+ * and its API token are configured here — edit this file when deploying.
+ *
+ * Security note: a real API token must NEVER be committed to the repository.
+ * The committed default below is intentionally empty; the deployer fills in
+ * the token at deploy time (and only then). The page logic reads the token
+ * from this module and never hardcodes one.
+ */
+
+/** The serverless endpoint the signup form POSTs to. */
+export const NEWSLETTER_ENDPOINT = "https://example.com/api/newsletter-subscribers";
+
+/** API token for the endpoint. Empty by default — set at deploy time. */
+export const NEWSLETTER_API_TOKEN = "";