setActiveExercise method
Implementation
void setActiveExercise(int index) {
if (_plan == null || _state == null) return;
if (index < 0 || index >= _plan!.exercises.length) return;
final already = _state!.activeExerciseIndex;
if (already != null) return;
final now = DateTime.now();
_activeExerciseIndex = index;
_state = _state!.copyWith(
activeExerciseIndex: index,
setIndex: 0,
updatedAt: now,
);
_persist();
notifyListeners();
}