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/blueprints/blocks/heading.yml

@@ -13,6 +13,9 @@ fields:
       - value: h3
         icon: h3
         text: H3
+      - value: h4
+        icon: h4
+        text: H4
   text:
     label: field.blocks.heading.text
     type: writer

site/blueprints/pages/report.yml

@@ -44,11 +44,14 @@ tabs:
             type: layout
             layouts:
               - "1/1"
+              - "1/2, 1/2"
             fieldsets:
               - text
               - heading
               - image
+              - video
               - beforeafter
+              - horizontalgallery
       - width: 1/6
         fields:
           emptyRight: