codepawan-dev/user-authentication-form ? reverse-engineered prompt

Build me a simple login and sign up web app with a clean, modern form UI. I want users to be able to create an account, log in, and see clear success or error messages when something goes wrong, like a bad password or missing fields. Please use Express for the server, MongoDB with Mongoose for storing users, Pug for the pages, and bcrypt for password hashing.

Keep it easy to run locally with one main app file, and make sure the forms look nice on desktop and mobile. Add the basic page flow for sign up, log in, and a simple success screen after login. If you need to check current docs for anything, go ahead and look them up online.

Please also organize the code so it’s easy to understand, with the static styles and page templates separated cleanly.