Fix scroll: keep message top visible, add scroll-down arrow; fix WS dupe error (v1.0.7)

Scroll behaviour:
- updateStreamContent no longer calls scrollToBottom() on every token.
  The top of the new message stays in view as content streams in.
- A sticky scroll-down arrow button appears at the bottom of the messages
  area whenever the user is >80px from the bottom. Clicking it smooth-
  scrolls to the bottom. Uses ResizeObserver to re-evaluate as content
  grows during streaming.

WebSocket duplicate error:
- When the Realtime API rejects session.update with an error event,
  onErrorCb is now nulled out before calling disconnect(). This prevents
  the subsequent WebSocket close event from firing a second "WebSocket
  closed: code …" message in the chat.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: caswelltom