fix(notifications): ensure schedule_at field has default value on submit

- Add default value of 0 for schedule_at field when not provided
- Restructure credentials object to explicitly set schedule_at before mutation
- Prevent undefined values from being sent to the API when schedule_at is empty

src/components/forms/notifications/useCreateNotificationFormPresenter.ts

@@ -38,7 +38,11 @@ const useCreateNotificationFormPresenter = () => {
     reset();
   });
   const onSubmit: SubmitHandler<TCreateNotificationCredentials> = (data) => {
-    mutate({ credentials: data });
+    const credentials = {
+      ...data,
+      schedule_at: data.schedule_at || 0,
+    };
+    mutate({ credentials });
   };
   const handleCancel = () => {
     reset();