[web] Fix filter panel showing over the topbar
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