1cca485062
Refactor the metric gradient code into shared _computeProgress + _buildGradient helpers. Hovering any of speed/HR/power/elevation/cadence stats switches the track to a blue→green→yellow→red gradient for that metric. A small legend (min ·gradient bar· max + label) appears in the bottom-left corner of the map while active. Absolute elevation used (not slope), so blue=valleys, red=peaks.