claude5opus/Claude-Opus-5-Free-Desktop ? reverse-engineered prompt

Build me a simple desktop app for Windows, macOS, and Linux that lets me chat with Claude Opus 5 in one place.

I want it to feel like a polished AI workbench, with a sidebar for sessions, a main chat area, a token counter, settings, and a markdown friendly response view. It should also have a built in model comparison panel so I can run the same prompt across Claude Opus 5, Sonnet 5, Fable 5, and older models, then compare the answers and benchmark info side by side.

Please make it easy to set up, with a first run setup flow and a clear way to configure access. Add automatic fallback to Sonnet 5 if Opus 5 gets rate limited. It should support large context so I can paste in long codebases for debugging, refactoring, and architecture review.

Keep the UI clean and modern, and if you need current model details or docs, look them up online first.