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/package.json

@@ -0,0 +1,12 @@
+{
+  "name": "beforeafter-block",
+  "version": "1.0.0",
+  "scripts": {
+    "dev": "npx -y kirbyup src/index.js --watch",
+    "build": "npx -y kirbyup src/index.js"
+  },
+  "devDependencies": {
+    "@vitejs/plugin-vue": "^6.0.1",
+    "vite": "^7.1.7"
+  }
+}