Build the History & Stats Screen

Read plan.md. Build milestone: Workout History.

Saving:
- On workout complete → save to localStorage: {id, date, preset, rounds, totalTime, completed: true}
- On stop early → save with completed: false

Add tab navigation in header: ⏱ Timer | 📊 History

History screen:
- Weekly heatmap: 7 boxes (Mon-Sun), green=completed day, orange=partial, purple border=today
- Stats row: 3 cards — streak (days), total sessions, total minutes
- Bar chart: weekly minutes for last 7 weeks
- Session list: cards with date, preset name, rounds, duration, done/partial badge
- Fade-in animation on tab switch