Add Stripe payment integration
- Integrate flutter_stripe SDK for payment processing
- Create StripeService with payment sheet flow
- Add tip selection UI (0%, 15%, 18%, 20%, 25%)
- Display fee breakdown (subtotal, tax, tip, service fee, card fee)
- Update cart screen with "Pay $X.XX" button
- Change MainActivity to FlutterFragmentActivity for Stripe compatibility
- Update Android themes to AppCompat for payment sheet styling
Fee structure:
- 5% Payfrit service fee (customer pays)
- 2.9% + $0.30 card processing fee (customer pays)
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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