Fix MERN Signup/Login Validation

I have an e-commerce site running on the classic MERN stack—MongoDB, Express, React, and Node—and the signup / login flow needs a quick but solid repair. Right now the pages render fine, yet the way they treat user data is off: the email field is slipping through with invalid formats, so users end up registered with bad addresses and then fail to log in.

Here’s what I need from you:

• Trace the current validation logic on both the client-side React form and the Express/Node API.
• Implement (or replace) email validation so only properly formatted addresses reach MongoDB. Regex or a library such as validator.js is fine as long as it’s lightweight and reliable.
• Make sure the fix doesn’t break existing password and username checks.
• Return clean, well-commented code and a brief note explaining what was changed so I can merge it quickly.

Once I can create an account with a valid email, see the user document stored correctly, and then log in without hiccups, the job is done.