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/SearchableTextBlock.svelte

@@ -160,7 +160,7 @@
 
 <div
   class="searchable-text flex flex-col gap-0 border border-stein-200 bg-stein-50 {layoutClasses} {className}"
-  data-block-type="searchable_text"
+  data-block="SearchableText" data-block-type="searchable_text"
   data-block-slug={block._slug}
 >
   <div class="p-4 md:p-6">