Hooray!!! Your good question provided the clue I needed: Just now checked my password manager in my browser and discovered I had saved my password with a typo.

Once I used the saved password, I was able to log into the desktop app with no problems.
Thank you (again) immensely, @WhiteNoise, for your perseverance and patience with my confusion. Genuinely appreciated, especially since this will now allow me to start playing with all the great publish features! 