Add mean lines for each treatment to `baseline_summary` and `covariate_summary`

There would be an extra line (grey dotted?) for each treatment within each treatment block (i.e. not the full height). Ideally implement it with the option to include/exclude.

Start by locating the code that renders `baseline_summary` and `covariate_summary`, then inspect how treatment blocks and plot options are built. The work is done when each treatment block can show its own mean line and users can include or exclude those lines through an option.

Written by the indexing model from the issue text.