billyphosly4/paystack-intergration ? reverse-engineered prompt

Build me a simple Paystack checkout app that uses plain HTML, CSS, vanilla JavaScript, and a small Node and Express backend.

I want a clean page where someone can enter payment details, click a button to pay, and be taken through Paystack checkout. After payment, the app should confirm whether the transaction was successful or not, and show a clear message on the page. Please include the backend routes needed to start the payment and verify the transaction using Paystack, plus the frontend code to call them.

Make it easy to run locally with npm start, and make sure it can also be deployed on Vercel. Use environment variables for the public and secret keys, and do not hardcode any secrets. If anything in the current Paystack docs has changed, look it up online and use the current recommended approach.