ChatGPTNextWeb/NextChat ? reverse-engineered prompt

I want to build my own AI chat assistant that's fast, clean, and works everywhere. The main thing is that it should be a single interface where I can connect to different AI models like GPT-4, Claude, and Gemini, just by plugging in my own API key. It needs to be very privacy-focused, so all the chat history should be stored locally in the user's browser, not on a server.

It should be a modern web app, but also a cross-platform desktop app for Mac, Windows, and Linux from the same codebase. For the tech, let's use a modern stack like Next.js and TypeScript, and maybe Tauri to wrap it for desktop.

Make sure the UI is responsive, has a dark mode, and can properly display formatted markdown and code blocks. I also want a feature where users can create, save, and reuse their own custom prompt templates. Finally, it should be simple for someone to self-host, so make it easy to deploy on Vercel.