feat: render actual elements in markdown cheatsheet instead of labels

Replace text labels (h1, bold, italic) with rendered HTML in the Rendu column:
headings, strong, em, del, code, links, blockquote, lists, hr, sup, small

app/public/admin/markdown-cheatsheet-fragment.php

@@ -0,0 +1,124 @@
+<?php
+/**
+ * Markdown cheatsheet — reusable modal dialog loaded via HTMX.
+ * Rendered as a <dialog> element; caller should call .showModal() after swap.
+ */
+
+$rows = [
+    [
+        'syntax' => '# Titre 1',
+        'render' => '<h1>Titre 1</h1>',
+        'note'   => 'Un # en début de ligne',
+    ],
+    [
+        'syntax' => '## Titre 2',
+        'render' => '<h2>Titre 2</h2>',
+        'note'   => 'Deux ## en début de ligne',
+    ],
+    [
+        'syntax' => '### Titre 3',
+        'render' => '<h3>Titre 3</h3>',
+        'note'   => 'Trois ### en début de ligne',
+    ],
+    [
+        'syntax' => '**gras**',
+        'render' => '<strong>gras</strong>',
+        'note'   => 'Double astérisque',
+    ],
+    [
+        'syntax' => '*italique*',
+        'render' => '<em>italique</em>',
+        'note'   => 'Simple astérisque',
+    ],
+    [
+        'syntax' => '~~barré~~',
+        'render' => '<del>barré</del>',
+        'note'   => 'Double tilde',
+    ],
+    [
+        'syntax' => '`code`',
+        'render' => '<code>code</code>',
+        'note'   => 'Backticks',
+    ],
+    [
+        'syntax' => '[lien](url)',
+        'render' => '<a href="#" class="md-cheatsheet-link">lien</a>',
+        'note'   => 'Texte entre crochets, URL entre parenthèses',
+    ],
+    [
+        'syntax' => '![image](url)',
+        'render' => '<span class="md-cheatsheet-img">🖼 image</span>',
+        'note'   => 'Point d\'exclamation + même syntaxe que lien',
+    ],
+    [
+        'syntax' => '> citation',
+        'render' => '<blockquote>citation</blockquote>',
+        'note'   => 'Chevron > en début de ligne',
+    ],
+    [
+        'syntax' => '- item',
+        'render' => '<ul><li>item</li></ul>',
+        'note'   => 'Tiret + espace',
+    ],
+    [
+        'syntax' => '1. item',
+        'render' => '<ol><li>item</li></ol>',
+        'note'   => 'Chiffre + point + espace',
+    ],
+    [
+        'syntax' => '---',
+        'render' => '<hr>',
+        'note'   => 'Triple tiret = ligne horizontale',
+    ],
+    [
+        'syntax' => '',
+        'render' => '',
+        'note'   => '',
+    ],
+    [
+        'syntax' => 'Texte avec [^1]',
+        'render' => 'Texte avec <sup>1</sup>',
+        'note'   => 'Appel de note de bas de page',
+    ],
+    [
+        'syntax' => '[^1]: La note.',
+        'render' => '<small>1. La note.</small>',
+        'note'   => 'Définition de la note (n\'importe où dans le document)',
+    ],
+];
+?>
+<dialog id="md-cheatsheet-dialog" class="md-cheatsheet-dialog">
+  <div class="md-cheatsheet-header">
+    <h2>Aide Markdown</h2>
+    <button type="button"
+            class="admin-icon-btn"
+            onclick="this.closest('dialog').close()"
+            title="Fermer"
+            aria-label="Fermer">
+      <svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" fill="currentColor" viewBox="0 0 256 256"><path d="M205.66,194.34a8,8,0,0,1-11.32,11.32L128,139.31,61.66,205.66a8,8,0,0,1-11.32-11.32L116.69,128,50.34,61.66A8,8,0,0,1,61.66,50.34L128,116.69l66.34-66.35a8,8,0,0,1,11.32,11.32L139.31,128Z"></path></svg>
+    </button>
+  </div>
+
+  <table class="md-cheatsheet-table">
+    <thead>
+      <tr>
+        <th>Syntaxe</th>
+        <th>Rendu</th>
+        <th>Notes</th>
+      </tr>
+    </thead>
+    <tbody>
+      <?php foreach ($rows as $row): ?>
+      <?php if ($row['syntax'] === ''): ?>
+      <tr class="md-cheatsheet-separator"><td colspan="3"></td></tr>
+      <?php else: ?>
+      <tr>
+        <td class="md-cheatsheet-syntax"><code><?= htmlspecialchars($row['syntax']) ?></code></td>
+        <td class="md-cheatsheet-render"><?= $row['render'] ?></td>
+        <td class="md-cheatsheet-note"><?= htmlspecialchars($row['note']) ?></td>
+      </tr>
+      <?php endif; ?>
+      <?php endforeach; ?>
+    </tbody>
+  </table>
+</dialog>