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/ImageBlock.svelte

@@ -40,7 +40,7 @@
   );
 </script>
 
-<div class={layoutClasses} data-block-type="image" data-block-slug={block._slug}>
+<div class={layoutClasses} data-block="Image" data-block-type="image" data-block-slug={block._slug}>
   {#if rawSrc}
     <figure class="max-w-[400px]">
       <RustyImage