- Increase near-bottom threshold from 200px to 450px so one long message
doesn't falsely register as 'near bottom'
- Make force=true only override on initial load (chatHasAutoScrolled=false),
not on subsequent refreshChat() calls
- refreshChat() no longer passes force=true to scheduleChatScroll
- Add chatNewMessagesBelow flag for future 'scroll to bottom' button UI
- Clear chatNewMessagesBelow when user scrolls back to bottom
- Add 13 unit tests covering threshold, force behavior, streaming, and reset