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/snippets/blocks/white-paper.php

@@ -0,0 +1,26 @@
+<?php
+/** @var \Kirby\Cms\Block $block */
+
+$wp = $block->whitePaper()->toPage();
+if (!$wp) return;
+
+$cover     = $wp->cover()->toFile()?->url();
+$bgColor   = $block->bgColor()->isNotEmpty()   ? $block->bgColor()->value()   : '#ffffff';
+$textColor = $block->textColor()->isNotEmpty()  ? $block->textColor()->value() : '#000000';
+$style     = 'background:' . htmlspecialchars($bgColor, ENT_QUOTES, 'UTF-8') . ';color:' . htmlspecialchars($textColor, ENT_QUOTES, 'UTF-8');
+?>
+<article class="wp-block" style="<?= $style ?>">
+  <div class="wp-block__content">
+    <p class="wp-block__label">Livre blanc</p>
+    <h3 class="wp-block__title"><?= html($wp->title()) ?></h3>
+    <?php if ($wp->intro()->isNotEmpty()): ?>
+      <p class="wp-block__intro"><?= strip_tags($wp->intro()->value()) ?></p>
+    <?php endif ?>
+    <button class="button with-icon download-icon wp-block__btn" type="button" data-uri="<?= htmlspecialchars($wp->uri(), ENT_QUOTES, 'UTF-8') ?>">
+      Téléchargement
+    </button>
+  </div>
+  <?php if ($cover): ?>
+    <img class="wp-block__cover" src="<?= htmlspecialchars($cover, ENT_QUOTES, 'UTF-8') ?>" alt="<?= html($wp->title()) ?>" loading="lazy" />
+  <?php endif ?>
+</article>