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/package.json

@@ -0,0 +1,12 @@
+{
+  "name": "video-block",
+  "version": "1.0.0",
+  "scripts": {
+    "dev": "npx -y kirbyup src/index.js --watch",
+    "build": "npx -y kirbyup src/index.js"
+  },
+  "devDependencies": {
+    "@vitejs/plugin-vue": "^6.0.1",
+    "vite": "^7.1.7"
+  }
+}