fix(seo): make og:image / twitter:image / JSON-LD image absolute
A post cover arrives as a relative /media/... path. Mastodon resolves it against the page URL,
but the OGP spec wants an absolute URL and stricter scrapers (WhatsApp, Signal, some fediverse
clients) won't resolve a relative one → no preview image. Absolutize _socialImage against the
canonical origin so every link/preview gets the real cover. (The AP Note image was already
absolute.)
- src/views/shell.ejs — absolutize _socialImage when it's a root-relative path
Co-Authored-By: Claude <noreply@…>