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/BlockSettings.tsx

@@ -12,7 +12,7 @@ import { useDevice } from '../DeviceToggle';
  *
  * ── Why it is not inside the free text ──
  *
- * Typing a heading into the text works for one kind of block and no others. A gallery, a video and a
+ * Typing a heading into the text works for one kind of block and no others. A carousel, a video and a
  * partner row all want to announce themselves and have nowhere to put the words — so the heading is a
  * property of the BLOCK rather than of its contents, and every kind gets one.
  *