Replace .admin-alert BEM classes with semantic role/data-type attributes

- admin.css: replace .admin-alert / .admin-alert--error / .admin-alert--success
  selectors with [role="alert"][data-type="error"] and [role="status"][data-type="success"]
- All 10 admin templates updated: <div class="admin-alert admin-alert--{type}">
  becomes <p role="alert|status" data-type="error|success"> (or <div> for the
  import.php multi-item list that contains a <ul>)
- flash-messages.php partial updated to match
- WCAG benefit: role="alert" is an ARIA live region — errors are announced
  immediately by screen readers without focus movement (fixes WCAG 3.3.1, 4.1.2)
- role="status" (polite live region) used for success messages — announced
  without interrupting the user
- Removes two BEM modifier classes; CSS now targets element semantics directly

public/admin/pages.php

@@ -25,7 +25,7 @@ unset($_SESSION['success']);
     <h1>Pages statiques</h1>
 
     <?php if ($success): ?>
-    <div class="admin-alert admin-alert--success">✓ <?= htmlspecialchars($success) ?></div>
+    <p role="status" data-type="success">✓ <?= htmlspecialchars($success) ?></p>
     <?php endif; ?>
 
     <table>