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/horizontal-gallery/index.php

@@ -0,0 +1,10 @@
+<?php
+
+Kirby::plugin('index/horizontal-gallery', [
+    'blueprints' => [
+        'blocks/horizontalgallery' => __DIR__ . '/blueprints/blocks/horizontalgallery.yml'
+    ],
+    'snippets' => [
+        'blocks/horizontalgallery' => __DIR__ . '/snippets/blocks/horizontalgallery.php'
+    ]
+]);