[web] Fix "See more..." button alignment in ChatThreadListSeeMoreSidebars component

Summary:
Address issue where the "See more..." button wasn't aligned with the rest of the items in the ChatThreadList.

More context: https://linear.app/comm/issue/ENG-920/see-more-threads-button-doesnt-look-right-on-web

Test Plan: Ensured that thread list looked correct visually. Tested in Chrome/Safari at different breakpoints.

Reviewers: atul

Subscribers: ashoat, palys-swm, Adrian, karol-bisztyga, benschac

Differential Revision: https://phabricator.ashoat.com/D3541