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/user_management_panel.dart

@@ -1,5 +1,5 @@
 import 'package:flutter/material.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/widgets/admin/admin_management_widget.dart';
 import 'package:p_tits_pas/widgets/dashboard/am_dossier_create_modal.dart';
 import 'package:p_tits_pas/widgets/dashboard/assistante_maternelle_management_widget.dart';
@@ -385,7 +385,7 @@ class _UserManagementPanelState extends State<UserManagementPanel> {
         context: context,
         barrierDismissible: false,
         builder: (dialogContext) {
-          return const AdminUserFormDialog();
+          return const StaffUserFormModal();
         },
       );
 
@@ -403,7 +403,7 @@ class _UserManagementPanelState extends State<UserManagementPanel> {
         context: context,
         barrierDismissible: false,
         builder: (dialogContext) {
-          return const AdminUserFormDialog(
+          return const StaffUserFormModal(
             adminMode: true,
             withRelais: false,
           );