Add MoonPay as the primary card payment provider for rCart. MoonPay
uses HMAC-SHA256 signed URLs (no session API needed, no IP whitelisting).
Falls back to Transak if MoonPay keys aren't configured.
- shared/moonpay.ts: URL builder with HMAC signing, currency mapping
- New /api/payments/:id/card-session endpoint picks provider automatically
- Frontend uses unified startCardPayment() with multi-provider message handling
- Set MOONPAY_API_KEY + MOONPAY_SECRET_KEY to activate
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>