unnatigupta153/auth-mern-app ? reverse-engineered prompt

Build me a simple MERN app with user sign up and log in, using a React front end and a Node and Express API with MongoDB on the back end.

I want users to be able to create an account, log in, stay signed in with a token, and log out. After logging in, show a basic protected page so I can see the auth flow working end to end. Please include clean forms, basic validation, and friendly error messages for things like missing fields, bad passwords, or an existing email.

Set it up so the client and server can run separately in development, with the API on port 4040 and the React app on port 3000. If you need current setup details, look up the latest docs online. Make it look decent with simple styling, and keep the code organized so it would be easy to add more protected routes later, like a products page.