add Worker: review habits at day end

[Ehsan-Tal]

Define day end - I'm thinking either 0400 or 0200.

Schedule a worker to run this code - and a notification with a progress bar to signal them being reviewed.

Have it run at day end - ensure we Variabilize it so that in future we can save it as a pref key.

In fact, maybe make it a pref key. And then once tested, add a setter function.

[Ehsan-Tal]

oh, also add a section in the About Habitua explaining why day end is not when the clock strikes midnight.