fix(chat): refocus input after submit + force scroll on new messages

[blove]

Summary

• Input refocus: After pressing Enter/submit, the textarea regains focus so the user can immediately type the next message without clicking
• Force scroll on new messages: When a new message is sent or received (message count changes), instantly scroll to bottom regardless of scroll position. During streaming partial updates, only smooth-scroll if user is near bottom (<150px) to avoid interrupting reading.

Test plan

• Chat library tests pass
• Input refocus after submit
• Scroll to bottom on new message
• Don't interrupt scroll position during streaming partials