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.ts

@@ -123,12 +123,12 @@ export const SECTIONS: Section[] = [
       Carousels — an entity of their own, not a list hanging off a page.
 
       Above Media and pinned with it, because the two are the same kind of thing: the raw material the
-      site is built from, held on this machine. Media is the files; a gallery is a set of them somebody
+      site is built from, held on this machine. Media is the files; a carousel is a set of them somebody
       has chosen and put in an order, which is a decision worth keeping once rather than remaking on
       every page that wants it.
 
       `requires: 'connection'`, unlike Media beside it, and the difference is real rather than a
-      copy-paste. Media is a file browser over a folder on this disk and needs nothing else. A gallery
+      copy-paste. Media is a file browser over a folder on this disk and needs nothing else. A carousel
       is a ROW in the site's database — reading or writing one goes through the site's admin API, so
       this section is useless without the development server answering, and opening it there would be
       a screen where every action fails.