I want "Done" button disabled until all files are uploaded. How do I do that?

Very easy to do with some custom JavaScript:

const widget = uploadcare.openDialog(null, 'file', {
  multiple: true
});
let total = 0;
let total_done = 0;
widget.fileColl.onAdd.add(file => {
  document.querySelector('.uploadcare--preview__done').style.display = 'none'; 
  total++;
  file.done(() => {
    total_done++;
    if (total_done == total) {
      document.querySelector('.uploadcare--preview__done').style.display = 'block'; // show "next" button when all files was uploaded
    }
  });
});

Check it live here.