asmileet/MERN-AI-chatbot ? reverse-engineered prompt

Build me a simple AI chatbot app where I can type a message and get an instant response back from OpenAI in a clean chat interface.

I want a nice looking, mobile friendly frontend and a secure backend that keeps the API key in environment variables instead of exposing it in the browser. Use a MERN style setup with React on the front end, Node and Express on the back end, and MongoDB wired in so the app is ready for future chat history or saved conversations.

Make the chat feel smooth and modern, with a clear message area, a text input, a send button, and basic loading or thinking feedback while the AI is replying. Keep the design simple, fast, and responsive. Please set it up so I can run the backend and frontend separately, and make sure the code is organized cleanly enough that I can extend it later with things like login, dark mode, and saved chats. If you need to check current docs for OpenAI or React, go ahead and look them up.