Adjust timeline bar height with extra spacingPublic

I will adjust the timeline bar CSS in frontend/components/messages/MessageList.tsx to ensure it extends correctly when nested.

I will modify the styling logic to:

1. Remove h-full from the base classes.
2. Add h-full only to the non-nested state.
3. Use -inset-y-6 (which sets both top and bottom to -24px) for the nested state, removing the bottom-0 override.

This will allow the timeline bar to stretch 24px above and 24px below the container height when isNested is true.

I've updated frontend/components/messages/MessageList.tsx to handle the timeline bar height as requested.

When isNested is true, it now uses -inset-y-6 (without h-full or bottom-0), which makes the bar extend 24px above and 24px below the container.