Agents App: Investigate why padding/sizing calculations are different on Windows vs. MacOS

Expected: 
Sizing and padding for UI elements should be the same across platforms, accounting for small differences in the font used. 

Actual: 
There's a noticeable jump in things like the width of the sessions list on the left, and the empty state input box. 
Is this all due to the font being slightly different? Or is there something else causing platform differences?