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/index.php

@@ -71,7 +71,7 @@ document.addEventListener('DOMContentLoaded', () => {
 
     <!-- Maintenance mode toggle -->
     <?php $maintenanceOn = file_exists(APP_ROOT . '/storage/maintenance.flag'); ?>
-    <div class="admin-maintenance-bar <?= $maintenanceOn ? 'admin-maintenance-bar--active' : '' ?>">
+    <aside role="status" class="admin-maintenance-bar <?= $maintenanceOn ? 'admin-maintenance-bar--active' : '' ?>" aria-label="Statut du site">
         <?php if ($maintenanceOn): ?>
             <span>⚠ Mode maintenance <strong>activé</strong> — le site public est inaccessible.</span>
             <form method="post" action="actions/maintenance.php" style="display:inline;">
@@ -90,7 +90,7 @@ document.addEventListener('DOMContentLoaded', () => {
                 </button>
             </form>
         <?php endif; ?>
-    </div>
+    </aside>
 
     <!-- Stats (always reflects full DB, independent of active filters) -->
     <dl class="admin-stats">