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/plugins/beforeafter/index.php

@@ -0,0 +1,10 @@
+<?php
+
+Kirby::plugin('index/beforeafter', [
+    'blueprints' => [
+        'blocks/beforeafter' => __DIR__ . '/blueprints/blocks/beforeafter.yml'
+    ],
+    'snippets' => [
+        'blocks/beforeafter' => __DIR__ . '/snippets/blocks/beforeafter.php'
+    ]
+]);