Help for writing an app (running zones using Karvonnen method)

There are some hr zone stuff implemented in Gadgetbridge for adding to "Activity Tracks". So if we logged the current karvonen zone in the recorder log, maybe we could integrate that into the info displayed in Gadgetbridge.

Here is the code in Gadgetbridge

Here I'm working on adding Activity Track support for Bangle.js in Gadgetbridge

EDIT: As per https://github.com/espruino/BangleApps/pull/3068#issuecomment-1788584559, it's probably better to have gadgetbridge calculate hr zones from the heart rate data when parsing. The info is there so it's unnecessary to add another field with calculations.