Smoother chat on message send

Sending a message no longer makes the chat area briefly disappear and reappear. The message list now stays stable through the whole send — from the moment you hit enter to the agent's first word — instead of blanking out and redrawing a few times along the way.

• ›No more flicker or black flash when sending a message
• ›Message list stays put while the agent starts streaming
• ›Artifacts and file previews stay mounted instead of re-rendering every few seconds