feat(payment-links): Add payment link creation and management UI

- Add /pay/create page for generating payment links
- Add /pay/[id] page for viewing and paying links
- Add /pay/history page for managing user's payment links
- Implement PaymentLinkQR component with QR code generation
- Add copy, share, and download QR code functionality
- Support payment link status tracking (active/paid/expired/cancelled)
- Add expiration time countdown and view count display
- Implement payment verification and transaction tracking
- Add paymentLinks utility library with validation helpers
- Generate Stellar URIs for wallet app integration
- Support custom recipient addresses and optional amounts

Author: Adwaitbytes