fix(seo): honour SITE_URL for blog canonicals (#933)

A build with a custom SITE_URL failed seo-audit on the blog pages:
generate-blog.mjs hard-coded https://www.bentopdf.com, and the
hand-written blog/author-alam.html carries that origin literally while
generate-i18n-pages.mjs only rewrites top-level pages.

generate-blog.mjs now reads SITE_URL like its sibling scripts, and
enhance-seo-pages.mjs rebases the default origin in dist/blog/*.html
to SITE_URL (dist only, no-op for the default URL) through a small
side-effect-free helper, scripts/site-origin.mjs, covered by
src/tests/site-origin.test.ts.

scripts/site-origin.mjs

@@ -0,0 +1,19 @@
+/**
+ * The public origin the build writes into canonical / og:url / JSON-LD URLs.
+ * Self-hosters set SITE_URL at build time; the default is the official site.
+ */
+export const DEFAULT_SITE_URL = 'https://www.bentopdf.com';
+
+const DEFAULT_ORIGIN_RE = /https:\/\/www\.bentopdf\.com(?![\w.-])/g;
+
+/**
+ * Rewrites absolute URLs on the default origin to `siteUrl` (no trailing
+ * slash). Used for hand-written pages that carry the official origin
+ * literally, so a build with a custom SITE_URL emits canonicals on its own
+ * host. Other hosts, including look-alikes such as
+ * `https://www.bentopdf.com.example`, are left untouched.
+ */
+export function rebaseDefaultOrigin(html, siteUrl) {
+  if (siteUrl === DEFAULT_SITE_URL) return html;
+  return html.replace(DEFAULT_ORIGIN_RE, siteUrl);
+}