add white-paper block type for articles

- Kirby block blueprint with page reference + editable bg/text colors
- PHP snippet renders the card with accessible markup (article, h3, button)
- WhitePaperDialog.svelte: native dialog with download form (a11y: labels, fieldset, autocomplete, focus management)
- Article.svelte: click detection on .wp-block__btn + dialog mount

refs #49

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

site/blueprints/blocks/white-paper.yml

@@ -0,0 +1,20 @@
+name: Livre blanc
+icon: download
+
+fields:
+  whitePaper:
+    label: Livre blanc
+    type: pages
+    max: 1
+    query: site.find('livres-blancs').children
+    image:
+      cover: true
+      ratio: 4/3
+  bgColor:
+    label: Couleur de fond
+    type: color
+    default: "#ffffff"
+  textColor:
+    label: Couleur de texte
+    type: color
+    default: "#000000"

site/blueprints/pages/article.yml

@@ -72,6 +72,8 @@ tabs:
                     extends: blocks/video
                   jeu:
                     extends: blocks/jeu
+                  white-paper:
+                    extends: blocks/white-paper
 
       # Sidebar
       sidebar: