create ER diagram, refactor to conform to diagram, simplify session management

WorkoutsPlus/ActiveWorkout/ActiveWorkoutSessionControls.swift

@@ -63,8 +63,7 @@ struct ActiveWorkoutSessionControls: View {
 }
 
 #Preview("isWorkingOut = true") {
-		@Previewable @State var activeWorkoutSession = WorkoutSession()
-		activeWorkoutSession.workout = Workout.sampleData.first!
+		@Previewable @State var activeWorkoutSession = WorkoutSession(start: Workout.sampleData.first!)
 		
 		// For some reason the return keyword is required here to avoid the error "Type of expression is ambiguous without a type annotation"
 		return ActiveWorkoutSessionControls(session: $activeWorkoutSession)
@@ -72,12 +71,3 @@ struct ActiveWorkoutSessionControls: View {
 						Defaults.shared.isWorkingOut = true
 				}
 }
-
-#Preview("isWorkingOut = false") {
-		@Previewable @State var activeWorkoutSession = WorkoutSession()
-		
-		return ActiveWorkoutSessionControls(session: $activeWorkoutSession)
-				.onAppear() {
-						Defaults.shared.isWorkingOut = false
-				}
-}