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/ImageGalleryBlock.svelte

@@ -155,7 +155,7 @@
 
 <svelte:window onkeydown={onModalKey} />
 
-<div class={layoutClasses} data-block-type="image_gallery" data-block-slug={block._slug}>
+<div class={layoutClasses} data-block="ImageGallery" data-block-type="image_gallery" data-block-slug={block._slug}>
   {#if descriptionHtml}
     <div class="markdown max-w-none prose prose-zinc mb-4">
       {@html descriptionHtml}