Design the front end of a desktop AI chat app

Design the client side of a desktop chat app for an AI assistant: the conversation list, the chat view and the model's replies as they stream in. This is a front-end system design round, so the focus is the app's architecture rather than the servers. Cover how state is organized and kept in sync with the backend, how streamed responses are rendered without janking the UI, how long conversations stay fast, and what happens when the connection drops in the middle of a reply.