admin: semantic HTML pass — checkbox fieldset, landmarks, dl/dt, autocomplete, inline styles

checkbox-list.php partial:
- Replace outer <div>/<label> with <div>/<span class="admin-row-label"> + inner
  <fieldset class="admin-checkbox-group"><legend class="sr-only"> to satisfy
  WCAG 1.3.1 (group label for multi-checkbox rows without duplicating visible text)
- Replace <div class="admin-checkbox-list"> with <ul>; each checkbox wrapped in <li>

admin.css:
- Drop .admin-checkbox-list; add .admin-body fieldset.admin-checkbox-group rules
  (border/padding reset so it doesn't inherit jury-fieldset box styling)
- Extend form-row label rule to span.admin-row-label
- .admin-inline-form + .admin-inline-form { margin-top:.35rem } replaces inline style
- .admin-input--inline / .admin-select--inline get width:160px (was inline style)
- .admin-tags-count + table th sizing via :has() replaces th inline styles

login.php: wrap content in <main id="main-content"> (missing landmark)

account.php:
- <div class="admin-account-status"> → <dl>; __label <span> → <dt>
- <div class="admin-danger-zone__description"> → <p>

index.php: <div class="admin-maintenance-bar"> → <aside role="status" aria-label="Statut du site">

add.php / edit.php: autocomplete="name" on author field, autocomplete="email" on
contact field (WCAG 1.3.5 / input purpose)

tags.php: all inline style= attributes removed (width, text-align, margin-top,
display:inline); all moved to CSS classes

public/admin/login.php

@@ -26,6 +26,7 @@ $pageTitle = 'Connexion';
 <?php $isAdmin = true; $bodyClass = 'admin-body'; require_once APP_ROOT . '/templates/head.php'; ?>
     <?php include APP_ROOT . '/templates/header.php'; ?>
 
+    <main id="main-content">
     <div class="admin-login-wrap">
         <div class="admin-login-box">
             <h2>Administration</h2>
@@ -43,5 +44,6 @@ $pageTitle = 'Connexion';
             </form>
         </div>
     </div>
+    </main>
 
 <?php require_once APP_ROOT . '/templates/admin/footer.php'; ?>