Prompt: Build the "start workout" flow: user picks a plan, a new row is created in workout_sessions with status='in_progress', and they can log actual sets/reps/weight per exercise into session_exercise_logs, pre-filled from the plan's targets. Users should be able to enter heart rate, steps, and calories. Each session log should include date and time elapsed. On finish, set status='completed' and ended_at. Also let users start an ad-hoc workout with no plan. Handle the case where the app is closed mid-workout — the in-progress session should still be resumable.