Improve Freight Selection UI

[BenHesketh21]

Checklist

• I've searched the issue queue to verify this is not a duplicate feature request.
• I've pasted the output of kargo version, if applicable.
• I've pasted logs, if applicable.

Proposed Feature

It would be great if there was a clearer way to select Freight when in promotion mode.

Motivation

I may be in the minority but I find selecting a freight when in promotion mode quite awkward. I always end up clicking one of the subscriptions or the freight name which don't open the confirm yes or no section. It would be great if was a bit clearer and easier to see where we should click on the freight.

Suggested Implementation

I would add a small bar/button at the bottom of eligible freights when in promotion mode that a user can click to get the confirmation message and then trigger a promotion. This way its much easier to see where you are meant to click.