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/PageStructure.tsx

@@ -22,7 +22,7 @@ import { BandBlocks } from './BandBlocks';
  * ── The gesture is the one used everywhere else ──
  *
  * Grip on the left, the row is the drop target, the list rearranges as the pointer crosses it, one
- * write at the end carrying every id. Pages, gallery items and partners all do this; a fourth way to
+ * write at the end carrying every id. Pages, carousel items and partners all do this; a fourth way to
  * reorder something would be a fourth thing to learn.
  */
 export const PageStructure = ({