Merge trains UX improvements

What does this MR do and why?

Making a few UX improvements to the merge trains viz before we start to rollout to the public.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Line height improvement

Dynamic text for merged cars

How to set up and validate locally

1. Enable merge trains and the FF merge_trains_viz
2. Add an MR to a merge train (via merge request view) and click "View merge details" in the pipeline widget on the MR.

Related to #472135