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/PublishButton.tsx

@@ -11,7 +11,7 @@ import { useReady } from './useReady';
  *
  * ── Why they are in the header and not only in Administration ──
  *
- * Publishing is the last step of every edit — write something on Home, choose a gallery, add a drop,
+ * Publishing is the last step of every edit — write something on Home, choose a carousel, add a drop,
  * then make it public. It lived in one card at the bottom of one section, so finishing a change meant
  * leaving the thing you were working on to go and find the button. A person who does not know the
  * panel simply does not find it, and concludes their edit went live when it did not.