refactor(#155): option C — widgets partagés sans préfixe Admin.

Déplace les composants Admin* du dashboard partagé vers
widgets/dashboard/ (ChildDetailModal, AmEditModal, UserCard, Select*, …).
Conserve AdminManagementWidget et screens/administrateurs. Rename mécanique,
aucun changement de comportement.

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

frontend/lib/widgets/admin/gestionnaire_management_widget.dart

@@ -4,7 +4,7 @@ import 'package:p_tits_pas/screens/administrateurs/creation/gestionnaires_create
 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';
-import 'package:p_tits_pas/widgets/admin/common/admin_user_card.dart';
+import 'package:p_tits_pas/widgets/dashboard/user_card.dart';
 import 'package:p_tits_pas/widgets/admin/common/suppression_confirm_dialog.dart';
 import 'package:p_tits_pas/widgets/admin/common/user_list.dart';
 
@@ -141,7 +141,7 @@ class _GestionnaireManagementWidgetState
         final user = filteredGestionnaires[index];
         final isSelf =
             _currentUserId != null && _currentUserId == user.id;
-        return AdminUserCard(
+        return UserCard(
           title: user.fullName,
           fallbackIcon: Icons.assignment_ind_outlined,
           avatarUrl: user.photoUrl,