Replace edit view with activity graph + range slider for workout trimming
- Add ActivityGraphView (Swift Charts line/area graph with dimming overlays)
- Add RangeSliderView (two-thumb slider for selecting trim range)
- Rewrite WorkoutEditView: graph, slider, time labels, activity picker
- Expand HealthKitManager with HR/energy/distance queries and realistic sample data
- Add destructive action warning to save confirmation
- Remove WorkoutAddView (not a use-case)