Add a shared "Powered by SkyPress" footer to reading pages
Single posts ended with a bare "More from {publication}" line, and
publication index pages had no footer at all — they surfaced the RSS
feed as a small link under the title instead. Neither page attributed
the platform.
Introduce PublicationFooter.astro, shared by both reading pages: the
publication name plus its RSS feed on the left, a quiet "Powered by
SkyPress" link home on the right. It takes its data as props and pulls
in none of the browser-only editor packages, so it stays on the safe
read path.
On the publication page this replaces the under-title RSS link; on the
single post it replaces the old reader__foot. The RSS glyph is the one
already used on the publication page, so the icon stays consistent.