feat(front): câbler PATCH refus utilisateur depuis wizards validation (#110)

- UserService.refuseUser → /users/:id/refuser
- AM + famille : envoi motifs, onSuccess + refresh liste
- Famille : refus séquentiel pour chaque parent en_attente
- ValidationRefusForm : état isSubmitting (UX)

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

frontend/lib/widgets/admin/validation_am_wizard.dart

@@ -480,15 +480,12 @@ class _ValidationAmWizardState extends State<ValidationAmWizard> {
         children: [
           Expanded(
             child: ValidationRefusForm(
+              isSubmitting: _submitting,
               onCancel: widget.onClose,
               onPrevious: () => setState(() => _showRefusForm = false),
               onSubmit: (comment) {
-                if (!mounted) return;
-                ScaffoldMessenger.of(context).showSnackBar(
-                  const SnackBar(
-                      content: Text('Refus (à brancher sur l’API refus)')),
-                );
-                widget.onClose();
+                if (comment == null || comment.trim().isEmpty) return;
+                _refuserEnvoyer(comment.trim());
               },
             ),
           ),
@@ -496,4 +493,26 @@ class _ValidationAmWizardState extends State<ValidationAmWizard> {
       ),
     );
   }
+
+  Future<void> _refuserEnvoyer(String comment) async {
+    if (_submitting) return;
+    setState(() => _submitting = true);
+    try {
+      await UserService.refuseUser(widget.dossier.user.id, comment: comment);
+      if (!mounted) return;
+      widget.onSuccess();
+    } catch (e) {
+      if (!mounted) return;
+      ScaffoldMessenger.of(context).showSnackBar(
+        SnackBar(
+          content: Text(e is Exception
+              ? e.toString().replaceFirst('Exception: ', '')
+              : 'Erreur'),
+          backgroundColor: Colors.red.shade700,
+        ),
+      );
+    } finally {
+      if (mounted) setState(() => _submitting = false);
+    }
+  }
 }