Add before/after comparison block plugin

- Create custom block plugin with Vue 3 + Composition API
- Use kirbyup for building Vue SFC components
- Block allows selecting two images with slider comparison UI
- Preview shows images overlapped at 50% in the panel
- Add beforeafter block to report blueprint
- Update report template to use authors field
- Change text block heading level from 4 to 3

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

site/templates/report.php

@@ -29,7 +29,7 @@
     <div class="panel__header">
         <span class="icon"><?= svg('assets/icons/toc.svg') ?></span>
         <span class="text">Table des matières</span>
-         <span class="icon close"><?= svg('assets/icons/close.svg') ?></span>
+        <span class="icon close"><?= svg('assets/icons/close.svg') ?></span>
     </div>
 
     <div class="panel__content">
@@ -77,7 +77,7 @@
         <dl class="report__dl">
             <div class="dl__group">
                 <dt>Auteurs du rapport</dt>
-                <dd>INDEX Investigation</dd>
+                <dd><?= $page->authors() ?></dd>
             </div>
             <div class="dl__group">
                 <dt>Date du rapport</dt>