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/blueprints/blocks/beforeafter.yml

@@ -0,0 +1,35 @@
+name: Comparaison Avant/Après
+icon: images
+preview: beforeafter
+fields:
+  imageBefore:
+    label: Image "Avant"
+    type: files
+    multiple: false
+    layout: cards
+    query: page.images
+    uploads: false
+    help: Image affichée à gauche / dessous
+  imageAfter:
+    label: Image "Après"
+    type: files
+    multiple: false
+    layout: cards
+    query: page.images
+    uploads: false
+    help: Image affichée à droite / dessus
+  caption:
+    label: Légende
+    type: text
+    help: Légende commune aux deux images
+  text:
+    label: Texte accompagnant
+    type: writer
+    marks:
+      - bold
+      - italic
+      - link
+    nodes:
+      - bulletList
+      - orderedList
+      - paragraph