Which preconditioning are you talking about? For the cabin, I think the mobile app can have a notification that tells you that the cabin has reached the target temperature.
The battery heating that *might* happen during that cabin conditioning does not have any messages. You can see that it's being performed by the three "bacon" lines in the mobile app. The bacon will disappear when the car stops actively heating the battery.
The battery heating en route to a supercharger is different. That has an indication in the car's navigation. When the message is displayed, the car is actively heating. There should also be bacon icons in the mobile app if you look at it during that time.
EDIT: you don't need to wait for it to complete before leaving home... You can hop in and drive without conditioning at all. You battery will just be colder. It might provide partial, or no regen, it might provide less power... but you won't hurt the battery by driving.