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/common/user_list.dart

@@ -1,5 +1,5 @@
 import 'package:flutter/material.dart';
-import 'package:p_tits_pas/widgets/admin/common/admin_list_state.dart';
+import 'package:p_tits_pas/widgets/dashboard/user_list_state.dart';
 
 class UserList extends StatelessWidget {
   final bool isLoading;
@@ -28,7 +28,7 @@ class UserList extends StatelessWidget {
       child: Column(
         crossAxisAlignment: CrossAxisAlignment.stretch,
         children: [
-          AdminListState(
+          UserListState(
             isLoading: isLoading,
             error: error,
             isEmpty: isEmpty,