Define tray item height token

[afurm]

Summary

• Move tray item height from a local magic number into a global CSS token.
• Keep tray layout sizing consistent via --tray-item-height.

Testing

• Not run (CSS-only change).

[jorgemanrubia]

This makes sense to me. What do you think @andyra?

[andyra]

I'm not quite sure this is an improvement. We still have a magic (i.e. arbitrary) number, but now it's further away from where we use it. Since this variable is only used in trays.css, there's not any advantage to hoisting it up to _global.css (usually reserved for tokens that are used throughout the app).

I suggest we keep this as-is—unless, of course, I'm missing something!