finish the carousel rename in the panels

Two of these were live: PageBuilderPanel still compared kind === 'gallery' in
two places, so a carousel block's editor and its Choose button were both dead
branches. The type checker could not see them — both compare against string.

The rest is copy somebody reads: "Change gallery", "Name a new gallery",
"Reading the gallery", "Add to this gallery", Home's Gallery heading, and the
"Create Gallery" file action, now "Carousel from folder".

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

web/sections/SubjectFields.tsx

@@ -13,7 +13,7 @@ import type { SubjectRecord, SubjectSection } from './subjects';
  * The fields below the description come from the section — see subjects.ts. Everything above them is
  * shared because it is the same in both: what it is called, and where it lives.
  *
- * The gallery is not here. It is chosen beside the photographs it puts on screen, in the panel, where
+ * The carousel is not here. It is chosen beside the photographs it puts on screen, in the panel, where
  * the consequence of choosing it is visible — a dropdown in a column of text fields would be the one
  * control whose effect you cannot see.
  *