feat(#164): uniformise la modale staff (gestionnaire / admin).

Shell 930 + Validation*Field, dirty-state, StaffUserFormModal sous
widgets/dashboard/ ; retire AdminUserFormDialog.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/lib/widgets/dashboard/gestionnaire_management_widget.dart

@@ -1,6 +1,6 @@
 import 'package:flutter/material.dart';
 import 'package:p_tits_pas/models/user.dart';
-import 'package:p_tits_pas/screens/administrateurs/creation/gestionnaires_create.dart';
+import 'package:p_tits_pas/widgets/dashboard/staff_user_form_modal.dart';
 import 'package:p_tits_pas/services/auth_service.dart';
 import 'package:p_tits_pas/services/user_service.dart';
 import 'package:p_tits_pas/utils/staff_deletion_rights.dart';
@@ -74,7 +74,7 @@ class _GestionnaireManagementWidgetState
       context: context,
       barrierDismissible: false,
       builder: (dialogContext) {
-        return AdminUserFormDialog(initialUser: user);
+        return StaffUserFormModal(initialUser: user);
       },
     );
     if (changed == true) {