[web] Add hover states to "Focus"/"Background" tabs

Summary: Quick thing to make things feel more interactive: https://linear.app/comm/issue/ENG-952/introduce-hover-states-for-focus-and-background-tabs

Test Plan:
Looks as expected:

Reviewers: def-au1t, palys-swm, benschac

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3574