GPX upload for free rides to automatically estimate training load

Currently, when logging a free ride, only duration and perceived fatigue can be entered. It would be much more accurate if users could upload a GPX file for planned or completed free rides. JOIN could then automatically calculate the training load based on elevation gain and estimated average speed (derived from past rides), making the load estimation far more precise than the current manual input.

This would bring free rides closer to the accuracy of structured workouts in terms of how they influence the training plan