Replace browser alert/confirm dialogs with <dialog> modals

- admin/index.php: alert() → no-selection dialog; confirm() bulk actions → bulk-confirm/bulk-delete dialogs; confirm() single delete → delete-thesis dialog; removed redundant confirm on Dépublier (reversible action)
- admin/tags.php: confirm() merge/delete → merge-tag/delete-tag dialogs
- admin/acces-etudiante.php: confirm() delete link → delete-link dialog
- admin/acces.php: confirm() archive link → archive-link dialog
- admin/parametres.php: confirm() maintenance/delete-all → enable-maintenance/delete-all-tfe dialogs; admin password confirm() kept with TODO comment
- admin/account.php: admin password confirm() kept with TODO comment
- admin.css: add .admin-dialog--sm, .admin-dialog__alert, .admin-dialog__footer styles

app/templates/admin/account.php

@@ -74,6 +74,7 @@
                 dépendra uniquement de l'authentification nginx Basic Auth si elle est configurée.
             </small>
         </p>
+        <?php /* TODO: replace this browser confirm() with a proper <dialog> modal like the other confirmations */ ?>
         <form method="post" action="/admin/actions/account.php"
               onsubmit="return confirm('Supprimer le mot de passe PHP ? L\'accès admin ne sera protégé que par nginx Basic Auth.')">
             <input type="hidden" name="csrf_token" value="<?= htmlspecialchars($_SESSION['csrf_token']) ?>">