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/pages/article.yml

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