Extract pure resolvePageHeadMeta helper that derives title suffix,
canonical URL, and OpenGraph fallbacks from the active site config and
router pathname. PageHead now reads useSite() + useLocation() to
render per-subdomain metadata (e.g. " | Nessa", canonical on
nessa.freno.me) instead of hardcoding " | Michael Freno".
- Always emit a canonical link (explicit override wins; otherwise
https://<site.domain><pathname>)
- Default og:image to the site ogDefaultImage when not provided
- Add unit tests covering all five sites, canonical derivation, and
OpenGraph fallback behavior
