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/blueprints/blocks/text.yml

@@ -6,5 +6,5 @@ fields:
   text:
     type: writer
     headings:
-      - 4
+      - 3
     placeholder: field.blocks.text.placeholder

site/blueprints/pages/report.yml

@@ -8,19 +8,22 @@ tabs:
     label: Contenu
     icon: page
     columns:
-      - width: 2/6
+      - width: 1/6
+        fields:
+          emptyLeft:
+            type: gap
+      - width: 4/6
         fields:
           created:
             label: Première publication
             type: date
             display: DD / MM / YYYY
             default: today
-            width: 1/4
-      - width: 4/6
-        fields:
+            width: 1/2
           subtitle:
             label: Sous-titre
             type: text
+            width: 1/2
           chapo:
             label: Chapo
             type: writer
@@ -42,15 +45,21 @@ tabs:
             layouts:
               - "1/1"
               - "1/2, 1/2"
-              - "1/3, 1/3, 1/3"
             fieldsets:
-              - heading
               - text
               - image
+              - beforeafter
+      - width: 1/6
+        fields:
+          emptyRight:
+            type: gap
   metadataTab:
     label: Métadonnées
     icon: table
     fields:
+      authors:
+        label: Auteur(s)
+        type: tags
       incidentDate:
         label: Date de l'incident
         type: date