feat(seo): separate display headings and improve social metadata

www/src/utils/seo.ts

@@ -0,0 +1,9 @@
+/** Resolve social assets against the public site, never a preview host or canonical. */
+export function socialImageUrl(image: string | undefined, site: string): string | undefined {
+  if (!image?.trim()) return undefined;
+  const url = new URL(image, new URL('/', site));
+  if (!['http:', 'https:'].includes(url.protocol) || url.username || url.password) {
+    throw new Error(`Invalid social image URL: ${image}`);
+  }
+  return url.href;
+}