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/canvas.ts

@@ -60,7 +60,7 @@ export type CanvasComponent = {
   /*
     One optional field per kind that the panel reads back. Not `Record<string, unknown>`: every reader
     would then need a cast, and the point of naming them is that the compiler knows `free-text` holds
-    `html` and `gallery` holds `carouselId`.
+    `html` and `carousel` holds `carouselId`.
   */
   content?: { html?: string; carouselId?: number | null; url?: string; ratio?: number; crop?: string };
   padding?: Record<string, { x?: number; y?: number }>;