#95 og:image is relative (needs public origin for scrapers)

What

og:image is emitted as a relative URL (/og.png) in src/layout.rs.

Impact

Many social scrapers require an absolute URL; without --public-origin the preview image may be ignored. With a Pages project origin, it should be absolute.

Fix

When public_origin is set, emit <public_origin>/og.png and make og:url absolute too; otherwise keep the relative fallback and document the limitation. Escape/validate the origin as today.