feat(paykit): unified error handling with typed error codes

- Add defineErrorCodes utility for compile-time validated error codes
- PayKitError extends better-call APIError for automatic HTTP response mapping
- Define PAYKIT_ERROR_CODES with 20+ typed codes across all domains
- Replace all raw Error throws with PayKitError.from(httpStatus, errorCode)
- Replace APIError usage in resolve-customer with PayKitError
- Export error codes and types for SDK consumers