[web] Fix filter panel showing over the topbar

Summary:
Filter panel used to have a fixed height. This is no longer possible, because since D6842 the topbars height is dynamic.

Test Plan: Run web app, checked that the filter panel is now correctly positioned under the topbar in all cases.

Reviewers: michal, kamil, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6848