r-follador/CubeTrek ? reverse-engineered prompt

Build me a web app for managing outdoor GPS activities, mostly for hiking, trail running, skiing, and mountain biking. I want users to sign up, connect devices like Garmin, Polar, or Coros, and upload FIT or GPX tracks if needed. The app should turn each activity into something much nicer than a plain map, with a 3D terrain view, a normal 2D map, useful stats, and a yearly activity overview. It should also group and compare similar activities, and show a heatmap of all tracks on one map so people can explore where they’ve been. Please make it feel polished and easy to use, with clear activity pages, track details, and interactive charts. If you need to look up current docs for the mapping or 3D pieces, go ahead and do that.