sasilucky28-code/pulse-bmi-calculator ? reverse-engineered prompt

Build me a polished BMI calculator web app that feels production ready.

Users should be able to enter height and weight in either metric or imperial, then instantly see their BMI, their health category like underweight, normal, overweight, or obese, and a short helpful tip. Please make sure the BMI is calculated on the server, not trusted from the browser, and add good validation so bad inputs show clear errors.

I also want optional private history tracking, so a user can save calculations, view their past results in a simple list, and delete entries. Make the UI responsive, accessible, and nice to use, with both dark and light mode and a theme toggle. Keep the design clean and modern, and include a clear result card or gauge style display.

Use a React frontend with a Node and Express backend and MongoDB for storage. If you need to check current docs or best practices, look them up online and then wire everything together so it runs locally with a simple setup.