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/snippets/blocks/horizontalgallery.php

@@ -0,0 +1,38 @@
+<?php
+/** @var \Kirby\Cms\Block $block */
+$images = $block->images()->toFiles();
+$text = $block->text()->value();
+?>
+
+<div class="subsection-w-hscroll">
+
+    <div class="horizontal-scroll-spacer"></div>
+
+    <div class="horizontal-scroll">
+        <div class="horizontal-scroll-wrapper">
+            <?php foreach ($images as $image): ?>
+            <div class="horizontal-scroll-slide">
+                <div class="horizontal-scroll-slide__inner">
+                    <figure>
+                        <img src="<?= $image->url() ?>" alt="<?= $image->alt()->esc() ?>" />
+                    </figure>
+                    <?php if ($image->caption()->isNotEmpty()): ?>
+                    <p class="caption"><?= $image->caption()->html() ?></p>
+                    <?php endif ?>
+                </div>
+            </div>
+            <?php endforeach ?>
+        </div>
+
+        <div class="horizontal-scroll-button-prev"></div>
+        <div class="horizontal-scroll-button-next"></div>
+        <div class="horizontal-scroll-pagination"></div>
+    </div>
+
+    <?php if ($text): ?>
+    <div class="subsection-txt">
+        <?= $text ?>
+    </div>
+    <?php endif ?>
+
+</div>