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/pages.php

@@ -31,10 +31,10 @@ unset($_SESSION['success']);
     <table class="admin-table">
         <thead>
             <tr>
-                <th>Slug</th>
-                <th>Titre</th>
-                <th>Mis à jour</th>
-                <th>Action</th>
+                <th scope="col">Slug</th>
+                <th scope="col">Titre</th>
+                <th scope="col">Mis à jour</th>
+                <th scope="col">Action</th>
             </tr>
         </thead>
         <tbody>