vikramgangaji512/VikramGPT ? reverse-engineered prompt

Build me a ChatGPT style web app called SigmaGPT, with a simple modern chat interface where a user can sign in, ask questions, and see the assistant reply in a clean conversation thread.

I want it to feel like a lightweight ChatGPT clone made with a MERN style setup, with a backend API and a frontend UI working together. The app should let users send messages, keep the chat history on screen, and show responses from the AI model. If possible, make it work with OpenAI and also keep it flexible enough to use a local Ollama model for development, so I can test it without relying on the cloud all the time. Please make the experience polished, responsive, and easy to use on desktop and mobile.

Set up the project so it runs cleanly from the two app folders, and add whatever basic configuration, error handling, and loading states are needed to make it usable right away. If you need to look up current docs online, go ahead.