You can link Codex and Claude right now (via their CLIs), and more providers will follow soon.
Tech-wise, this stared as a native SwiftUI app, but it was actually terrible. Not that SwiftUI is terrible, but that my web development background didn't lend itself to getting a good result with it.
So it's now Tauri (rust backend, React frontend, using the system's built-in web view), which has been great. Turns out browsers are highly optimized for UI!