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/subjects.ts

@@ -6,7 +6,7 @@ import { Boxes, Shapes } from 'lucide-react';
  *
  * ── Why one shape ──
  *
- * They are the same object. A title, an address, some words, a gallery, and a table of properties;
+ * They are the same object. A title, an address, some words, a carousel, and a table of properties;
  * the sections differ in what they GROUP, not in how one is edited. Copying the panel would be three
  * hundred lines duplicated for an icon and a noun, and the copies would drift the first time one of
  * them got a fix.
@@ -25,7 +25,7 @@ import { Boxes, Shapes } from 'lucide-react';
 /** What every subject row has. Snake case: it is the wire, not our shape. */
 export type SubjectRecord = {
   id: number;
-  /** The gallery holding its media, or null while nobody has chosen one. */
+  /** The carousel holding its media, or null while nobody has chosen one. */
   carousel_id: number | null;
   title: string;
   slug: string | null;