Enhance chat message list with infinite scroll and viewport preservation - #51

- Introduce IntersectionObserver to detect scroll events for reaching the top and bottom of the message list.
- Add new props for loading states and viewport management, including `loadingNewer`, `centerOnMessageId`, and `preserveViewportOnPrepend`.
- Implement logic to emit events when the top or bottom of the message list is reached, allowing for dynamic loading of messages.
- Refactor message loading logic to support windowed message display based on read anchors and user interactions.
- Update unit tests to cover new scrolling behavior and viewport preservation features.

- Document changes to the chat timeline, including a bounded initial message window to improve loading experience.
- Note the transition from button-based pagination to bidirectional infinite scroll for chat history.
- Highlight the new initial message viewport behavior, centering around the user's last-read event when applicable.