Add scheduling to Opal ⏱️#4

It’d be great to be able to schedule when we want Opal to turn on/off in advance so we don’t have to remember to open the app every time we want to turn it on.

Saved preferences/modes:
I use Opal differently during the work day vs weeknight evening vs weekend. It would be nice if I could save settings (ex different default pause times, different apps disabled, etc.)