feat(blocks): enhance heading, video, and add gallery block

- Add H5 heading level option to heading block
- Improve video block fallback image help text
- Add new gallery block with image slider functionality

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

kirby/config/blocks/video/video.yml

@@ -34,8 +34,7 @@ fields:
       back: black
     uploads:
       template: blocks/image
-    when:
-      location: kirby
+    help: Affichée si la vidéo ne charge pas. **Obligatoire pour la version print.**
   caption:
     label: field.blocks.video.caption
     type: writer