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/storage/logs/admin.log

@@ -0,0 +1 @@
+{"timestamp":"2026-05-04T15:36:30+00:00","ip":"127.0.0.1","user_agent":"Mozilla/5.0 (X11; Linux x86_64; rv:150.0) Gecko/20100101 Firefox/150.0","resource":"system","action":"maintenance_on","status":"success"}

app/storage/maintenance.flag

@@ -0,0 +1 @@
+2026-05-04T15:36:30+00:00