Ah! I've been tinkering on a very similar project off and on for years now. This is so awesome to see— really nice work. I've just never had time to actually polish mine off. I think the backend for processing new activities is broken at the moment, but the old demo activities are still around.
Ex: https://contour.run/home?activityId=36948814953
I've been using three.js and react-three-fiber for it. happy to swap notes! nice work :)