Podcast USB v0.3.1

Closing while work is active now shows a confirmation with the current task
and live download/audio estimates. Options are Keep running, Finish then close,
and Cancel & close. Finish then close completes the work before exiting;
Cancel & close performs orderly cancellation before closing. The final USB
transaction cannot be interrupted. New jobs cannot start after shutdown is
requested. All v0.3 features and saved settings are retained.
