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/horizontalgallery.yml

@@ -1,22 +0,0 @@
-name: Galerie horizontale
-icon: images
-preview: horizontalgallery
-fields:
-  images:
-    label: Images
-    type: files
-    multiple: true
-    layout: cards
-    query: page.images
-    help: Pensez à ajouter une légende à chaque image via son blueprint (champ Caption)
-  text:
-    label: Texte accompagnant
-    type: writer
-    marks:
-      - bold
-      - italic
-      - link
-    nodes:
-      - bulletList
-      - orderedList
-      - paragraph