admin: semantic HTML improvements — dl stats, section cards, th scope

- admin/index.php: replace <div class="admin-stats"> with <dl>; inner
  <div class="admin-stat__number"> → <dd>, <div class="admin-stat__label"> → <dt>;
  use CSS order to keep number visually first; add scope="col" to all 9 <th> cells

- admin/thanks.php: replace all four <div class="admin-thesis-info"> wrappers
  with <section> elements; remove the class entirely; add scope="col" to
  the files table <th> cells

- admin/tags.php: add scope="col" to all 3 <th> cells

- admin/pages.php: add scope="col" to all 4 <th> cells

- admin.css: rename .admin-thesis-info selectors to .admin-main > section
  (element + context selector — no class needed); add display:flex +
  flex-direction:column to .admin-stat so CSS order property works correctly

Addresses TODO items: section X (admin-stats dl, th scope), XI (tags th scope),
XII (admin-thesis-info → section), XIII (pages.php th scope)

public/admin/thanks.php

@@ -73,7 +73,7 @@ $pageTitle = "Récapitulatif TFE";
         <p><a href="/admin/add.php" class="admin-btn-secondary">Retour au formulaire</a></p>
 
     <?php elseif ($thesis): ?>
-        <div class="admin-thesis-info">
+        <section>
             <h2>Informations de base</h2>
             <dl>
                 <dt>Identifiant</dt><dd><?= htmlspecialchars($thesis['identifier']) ?></dd>
@@ -84,9 +84,9 @@ $pageTitle = "Récapitulatif TFE";
                 <dt>Auteur·ice(s)</dt><dd><?= htmlspecialchars($thesis['authors']) ?></dd>
                 <dt>Année</dt><dd><?= htmlspecialchars($thesis['year']) ?></dd>
             </dl>
-        </div>
+        </section>
 
-        <div class="admin-thesis-info">
+        <section>
             <h2>Détails académiques</h2>
             <dl>
                 <dt>Orientation</dt><dd><?= htmlspecialchars($thesis['orientation'] ?? '–') ?></dd>
@@ -96,9 +96,9 @@ $pageTitle = "Récapitulatif TFE";
                 <dt>Promoteur·ice(s)</dt><dd><?= htmlspecialchars($thesis['supervisors']) ?></dd>
                 <?php endif; ?>
             </dl>
-        </div>
+        </section>
 
-        <div class="admin-thesis-info">
+        <section>
             <h2>Contenu</h2>
             <dl>
                 <?php if ($thesis['languages']): ?>
@@ -117,13 +117,13 @@ $pageTitle = "Récapitulatif TFE";
                 <dt>Lien</dt><dd><a href="<?= htmlspecialchars($thesis['baiu_link']) ?>" target="_blank" rel="noopener"><?= htmlspecialchars($thesis['baiu_link']) ?></a></dd>
                 <?php endif; ?>
             </dl>
-        </div>
+        </section>
 
         <?php if (!empty($files)): ?>
-        <div class="admin-thesis-info">
+        <section>
             <h2>Fichiers</h2>
             <table class="admin-table">
-                <thead><tr><th>Type</th><th>Fichier</th><th>Taille</th><th>Date</th></tr></thead>
+                <thead><tr><th scope="col">Type</th><th scope="col">Fichier</th><th scope="col">Taille</th><th scope="col">Date</th></tr></thead>
                 <tbody>
                     <?php foreach ($files as $f): ?>
                     <tr>
@@ -135,7 +135,7 @@ $pageTitle = "Récapitulatif TFE";
                     <?php endforeach; ?>
                 </tbody>
             </table>
-        </div>
+        </section>
         <?php endif; ?>
 
         <div style="margin-top:1.5rem;display:flex;gap:.75rem;flex-wrap:wrap;">