Loop remote carbs: allow future carbs up to 4 h for the future_carbs_4h customization #815

Users who build Loop with the Loop and Learn customization future_carbs_4h would like to enter remote carbs further ahead from LoopFollow.

future_carbs_4h raises LoopConstants.maxCarbEntryFutureTime in Loop from 1 h to 4 h. Loop validates remote carb entries against that same constant, so a customized Loop already accepts remote carbs up to 4 h ahead. LoopFollow caps the time at 1 h ahead (maxFutureHours in LoopAPNSCarbsView and LoopCarbEditSheet), which blocks this. An unmodified Loop still rejects anything beyond 1 h, so the limit needs to be a setting with 1 h as the default.