- add 'register' command: register account and print credentials only
- add 'checkout' command: register account and print Plus hosted checkout URL
- fix hCaptcha rqdata/isInvisible for Stripe setup intent verification
- fix verify_challenge flow: call chatgpt.com/checkout/verify after challenge
- fix currency uppercase for checkout API
- add get_checkout_url method to ChatGPTPayment
- switch default mail provider to yyds
- update README with new commands and usage