Summary

Adds support for HIIT-style autonomous timer flow on duration-based exercises:

1. Press play → set countdown begins
2. Countdown reaches 0 → rest timer auto-starts
3. Rest reaches 0 → next set countdown auto-starts
4. Repeats until all sets are complete (no taps in between)

This unlocks Tabata / EMOM / circuit workouts where the user wants hands-free progression through a fixed work/rest cycle.

Changes

Data layer

• Exercise: new fields targetDuration: Int and autoAdvanceSets: Boolean
• ExerciseDC: new field cautions: String (common-mistakes text shown to the user)
• Room MIGRATION_3_4 adds the three new columns (defaults preserve existing rows)
• UI models (UiExercise, UiExerciseDC) and their mappers updated to match

Service layer

• WorkoutService gains a countdown coroutine and four new StateFlows:
  countdownTime, isCountdownActive, countdownTotal, countdownFinished
• Two new actions: START_COUNTDOWN, CANCEL_COUNTDOWN
• When a countdown finishes, the service auto-starts the rest timer if a rest duration was provided
• WorkoutServiceManager exposes startCountdown(duration, restDuration) and cancelCountdown()

ViewModel

• WorkoutScreenViewModel.HiitPhase sealed class: Idle / SetCountdown / RestBetweenSets / ExerciseDone
• Observers tie the lifecycle together:
  • On countdownFinished → mark the set as completed, transition to RestBetweenSets
  • On rest timer reaching 0 while in RestBetweenSets → start the next set's countdown
• startHiitCountdown / cancelHiitCountdown / resetHiitPhase for screen control

UI

• New HiitCountdownCard composable with a circular progress arc and color-coded phase
  • orange = set, red = last 3 seconds, blue = rest, green = done
  • controls: Start Set / Cancel / Skip Rest / Info
• WorkoutScreen renders the card above the regular ExerciseCard for any exercise where
  setMode == DURATION && targetDuration > 0 && autoAdvanceSets
• InfoExerciseScreen shows a "Common Mistakes" warning card on the Instructions tab when
  cautions is non-empty (no-op for legacy exercises with empty cautions)

Backward compatibility

• All new entity fields default to 0 / false / "" so legacy workouts are unaffected
• Existing rest-timer behaviour is unchanged when autoAdvanceSets is false
• The HIIT card only appears for exercises that explicitly opt in

Test plan

• Fresh install: DB created at v4, app launches normally
• Upgrade from v3: migration runs, existing workouts open without crashes
• Create a DURATION exercise with targetDuration=30, restTime=15, autoAdvanceSets=true, 3 sets
  • Tap Start Set → arc counts 30 → 0 with orange color
  • Last 3 s turn red
  • Rest auto-starts in blue, counts 15 → 0
  • Next set begins automatically; cycle repeats for set 3
  • After final set, phase shows "DONE" / green
• Tap Cancel mid-countdown → returns to idle, set NOT marked complete
• Tap Skip Rest during rest → next set's countdown begins immediately
• Tap Info → navigates to InfoExerciseScreen
• Backgrounding the app during a countdown keeps the foreground notification alive
• Existing non-HIIT (LOAD / BODYWEIGHT) flow unaffected
• InfoExerciseScreen: stock exercises with empty cautions show no warning card; an exercise with cautions set shows the red warning card on the Instructions tab

Notes

UI to edit targetDuration / autoAdvanceSets from the Edit-Exercise screen is not included in this PR — the fields are wired through the data and service layers and the runtime UI, but a follow-up PR can add the form controls. For now they can be set programmatically or via custom-exercise creation. Happy to split this further or fold the edit UI in if maintainers prefer.