pass a copy when opening dialog

otherwise, any changes the dialog makes will reflect in the calling
view's object

dashboard/src/components/MailinglistDialog.vue

@@ -57,6 +57,7 @@ async function onSubmit() {
 
 defineExpose({
   async open(m = null) {
+    m = m ? JSON.parse(JSON.stringify(m)) : null; // make a copy
     busy.value = false;
     formError.value = {};
     mailinglist.value = m;