Refactor blocks architecture to modular approach

- Restore "1/2, 1/2" layout for flexible column combinations
- Simplify beforeafter block: remove toggle and text field, keep only image comparison
- Create new video block with URL support (YouTube/Vimeo/direct files)
- Create horizontal-gallery block for scrollable image galleries
- Add H4 heading level support
- All blocks now modular: combine with text blocks in 2-column layouts

Blocks available:
- Text, Heading (h2-h4), Image, Video
- Before/After comparison (no text)
- Horizontal gallery (with text below)

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

site/plugins/beforeafter/blueprints/blocks/beforeafter.yml

@@ -1,52 +1,23 @@
-name: Image avec texte
+name: Comparaison Avant/Après
 icon: images
 preview: beforeafter
 fields:
-  isBeforeAfter:
-    label: Mode comparaison Avant/Après
-    type: toggle
-    default: false
-  image:
-    label: Image
-    type: files
-    multiple: false
-    layout: cards
-    query: page.images
-    when:
-      isBeforeAfter: false
   imageBefore:
     label: Image "Avant"
     type: files
     multiple: false
     layout: cards
     query: page.images
-    uploads: false
     help: Image affichée à gauche / dessous
     width: 1/2
-    when:
-      isBeforeAfter: true
   imageAfter:
     label: Image "Après"
     type: files
     multiple: false
     layout: cards
     query: page.images
-    uploads: false
     help: Image affichée à droite / dessus
     width: 1/2
-    when:
-      isBeforeAfter: true
   caption:
     label: Légende
     type: text
-  text:
-    label: Texte
-    type: writer
-    marks:
-      - bold
-      - italic
-      - link
-    nodes:
-      - bulletList
-      - orderedList
-      - paragraph