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/DetailPanel.tsx

@@ -31,7 +31,7 @@ import { useReady } from './useReady';
  * ── Not ready means Administration, whatever the URL says ──
  *
  * The gate is `useReady`, shared with the nav so the two cannot disagree. Enforced HERE and not only
- * there, because the nav can only decline to offer a link — `?section=gallery` typed into the address
+ * there, because the nav can only decline to offer a link — `?section=carousels` typed into the address
  * bar, or a bookmark from before the connection was forgotten, arrives without going through it. A
  * nav that hides a door is a courtesy; this is the lock.
  */