import/restore: check validity after prefill

dashboard/src/views/RestoreView.vue

@@ -303,6 +303,8 @@ function onBackupConfigChanged(event) {
         providerConfig.value[key] = value;
       }
     }
+
+    setTimeout(checkValidity, 100); // update state of the confirm button
   };
 
   reader.readAsText(event.target.files[0]);