This is a really great article. Thank you very much for writing it. It may be helpful to consider the following change

executor.execute(() -> {
    if (response != null) {
        User user = response.body();
        if (user != null) {
            user.setLastRefresh(new Date());
            userDao.save(user);
        }
    }
});

If the USER_LOGIN on line 22 of the MainActivity is changed to an unrecognized username, the app will crash on an NPE.

Obviously, you put a lot of time and effort into the code and this article. Thank you very much for your hard work.