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/video/blueprints/blocks/video.yml

@@ -0,0 +1,11 @@
+name: Vidéo
+icon: video
+preview: video
+fields:
+  url:
+    label: URL de la vidéo
+    type: url
+    help: URL YouTube, Vimeo ou lien direct vers un fichier vidéo
+  caption:
+    label: Légende
+    type: text