Rename horizontalgallery block to horizontal-gallery

- Follow Kirby naming convention with dashes
- Update all references (blueprint, snippet, plugin registration)
- Rebuild plugin with new name

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

site/plugins/horizontal-gallery/blueprints/blocks/horizontal-gallery.yml

@@ -1,6 +1,6 @@
 name: Galerie horizontale
 icon: images
-preview: horizontalgallery
+preview: horizontal-gallery
 fields:
   images:
     label: Images