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/OrganisationsBlock.svelte

@@ -129,7 +129,7 @@
   </Card>
 {/snippet}
 
-<div class={layoutClasses} data-block-type="organisations" data-block-slug={block._slug}>
+<div class={layoutClasses} data-block="Organisations" data-block-type="organisations" data-block-slug={block._slug}>
   {#if block.headline}
     <h3
       class={compact ? "mb-2 text-lg font-semibold text-stein-900" : "mb-4"}