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>

site/blueprints/blocks/gallery.yml

@@ -0,0 +1,35 @@
+name: Galerie (slider)
+icon: dashboard
+preview: gallery
+fields:
+  images:
+    label: field.blocks.gallery.images.label
+    type: files
+    query: model.images
+    multiple: true
+    layout: cards
+    size: small
+    empty: field.blocks.gallery.images.empty
+    uploads:
+      template: blocks/image
+    image:
+      ratio: 1/1
+  # ratio:
+  #   label: field.blocks.image.ratio
+  #   type: select
+  #   placeholder: Auto
+  #   width: 1/2
+  #   options:
+  #     1/1: "1:1"
+  #     16/9: "16:9"
+  #     10/8: "10:8"
+  #     21/9: "21:9"
+  #     7/5: "7:5"
+  #     4/3: "4:3"
+  #     5/3: "5:3"
+  #     3/2: "3:2"
+  #     3/1: "3:1"
+  # crop:
+  #   label: field.blocks.image.crop
+  #   type: toggle
+  #   width: 1/2