chat-composer.

the line you type into, under a chat-log. split from the log rather than bundled with it because the two have different reasons to re-render — the log repaints on every arriving message, the composer only on your own keystrokes — and because a read-only room is a real state: render the log without this and there is nothing to disable or explain. owns the draft and nothing else; onSend may be async, and a rejected send hands the sentence back rather than losing it, because losing a typed sentence to a dropped request is the one failure a chat box must not have.