- Create BiometricService with local_auth integration
- Check biometric availability (Face ID, Touch ID, fingerprint)
- Authenticate with fallback to PIN
- Session state: don't re-prompt within session
- Configurable lock policies: always, after 5 min inactive, never
- Create AuthGate widget that wraps the app
- Shows biometric prompt on app launch
- Handles app resume from background
- Activity tracking for inactivity timeout
- Graceful error handling with user-friendly messages
- Update SettingsScreen with biometric configuration
- Enable/disable toggle with verification
- Lock timing policy selection
- Edge case handling: not enrolled, not available
- Store preferences in shared_preferences
- Handle multiple failures with attempt counter