docs: add complete Integration documentation

Created comprehensive integration guides:

- frontend.md: React/TypeScript integration with Freighter wallet, authentication,
  invoice creation, payment flow, React Query, error handling, complete examples

- backend.md: Node.js/Express integration, API client, authentication, request
  validation, payment verification, webhooks, error handling, database integration

- webhooks.md: Webhook events (invoice.paid, invoice.expired, link.paid, etc.),
  signature verification, security best practices, retry logic, complete handlers

- authentication.md: Passwordless wallet authentication, ed25519 signatures,
  nonce-based auth flow, JWT tokens, production considerations, security

All guides include:
- Complete code examples
- Security best practices
- Error handling
- Testing strategies
- Troubleshooting
- Mermaid diagrams

Total: 4 integration guides (3,000+ lines)

Author: carlos-israelj