Adds a swipeable 3-slide onboarding screen shown on first launch:
1. "Meet Timmy" — introduces the AI wizard with animated face
2. "Talk, Don't Type" — explains voice interaction
3. "Lightning Fast Payments" — explains pay-per-request model
Uses AsyncStorage to persist completion. Subsequent launches skip
straight to the main tab navigator. Users can skip or tap through.
Fixes#35
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
