Previously the chart rendered nothing at all unless a structured Garmin
workout's target was resolved, which meant most activities showed no chart.
Now it always shows actual pace/HR over elapsed time (in minutes) for any
activity with laps, filled as an area rather than a line, with the expected
band layered on top only when available. Each lap's warm-up/effort/recovery/
cool-down phase (from Garmin's per-lap IntensityType) shades the background,
with a small legend for whichever phases are present.
Activities recorded from a structured Garmin workout carry a workoutId;
when its steps line up 1:1 with the recorded laps, the per-step target
pace/HR zone is resolved (via a Karvonen lookup for named zones) and stored
on each lap. The Review Queue plots it against the actual per-lap pace/HR
so the user can eyeball whether a run matches its plan while sorting it.
