feat(blocks): data-block PascalCase identifier on every CMS block

Adds data-block="PascalName" alongside existing data-block-type
snake_case so blocks can be located via [data-block="Calendar"] etc.
in DevTools. Snake-case attribute kept for backward compat.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/lib/components/blocks/YoutubeVideoGalleryBlock.svelte

@@ -145,7 +145,7 @@
 
 <div
   class={layoutClasses}
-  data-block-type="youtube_video_gallery"
+  data-block="YoutubeVideoGallery" data-block-type="youtube_video_gallery"
   data-block-slug={block._slug}
 >
   {#if block.title}