The scroll-to-bottom functionality wasn't working properly on page load because
messages take time to load and render. This fix adds:
- Enhanced scrollChatToBottom() with image load detection
- ResizeObserver to detect content height changes
- Multiple scroll attempts with increasing delays (up to 2s)
- Additional scroll calls in selectSessionById with proper delays
- Scroll calls at end of initBuilder initialization
This ensures the chat area properly scrolls to the bottom even when content
loads asynchronously or images take time to render.