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/assistante_maternelle_management_widget.dart

@@ -5,8 +5,8 @@ import 'package:p_tits_pas/utils/am_vigilance.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';
-import 'package:p_tits_pas/widgets/admin/common/admin_am_edit_modal.dart';
-import 'package:p_tits_pas/widgets/admin/common/admin_user_card.dart';
+import 'package:p_tits_pas/widgets/dashboard/am_edit_modal.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';
 
@@ -130,7 +130,7 @@ class _AssistanteMaternelleManagementWidgetState
       itemBuilder: (context, index) {
         final assistante = filteredAssistantes[index];
         final vigilance = amPlacesVigilanceMessage(assistante);
-        return AdminUserCard(
+        return UserCard(
           title: assistante.user.fullName,
           avatarUrl: assistante.user.photoUrl,
           fallbackIcon: Icons.face,
@@ -161,7 +161,7 @@ class _AssistanteMaternelleManagementWidgetState
   void _openAssistanteDetails(AssistanteMaternelleModel assistante) {
     showDialog<void>(
       context: context,
-      builder: (context) => AdminAmEditModal(
+      builder: (context) => AmEditModal(
         assistante: assistante,
         onSaved: _loadAssistantes,
       ),