Weekly focus stats screen with charts

[sonnymay]

Summary

The app tracks sessions, XP, and streaks but has no visual analytics screen. A weekly stats view would increase user engagement and retention by showing progress over time.

Acceptance criteria

• New "Stats" tab showing: total focus hours this week, sessions completed, current streak, best streak
• Bar chart showing daily focus minutes for the past 7 days (React Native Skia or Victory Native)
• "This week vs last week" comparison row showing delta for hours and sessions
• All data pulled from AsyncStorage session history — no backend required
• Screen handles empty state gracefully (first-time users)
• Animated number counters on mount (Reanimated)

Stack

react-native-skia or victory-native for charts · Reanimated for number animations