[REFACTOR]: Remove redundant message state in ChatContainer

[shatfield4]

What would you like to see?

• ChatContainer maintains a message state that syncs with PromptInput on every keystroke via onChange
• This is redundant since PromptInput already owns its own input state internally
• This causes ChatContainer to rerender on every keystroke, which cascades to child components unnecessarily
• The recent fix in fc9bd71 addressed a visual symptom of this (animation restart) by stabilizing the key prop, but the underlying unnecessary rerenders remain