Reset footerHeight on BottomBarWeb unmount
When the window expands past the mobile breakpoint, BottomBarWeb
unmounts and DesktopLeftNav renders instead. However footerHeight
(a SharedValue set by onLayout) remained at ~50px from the prior
render. This stale value caused the message input translateY to
pull it over the message list at intermediate breakpoints where
the bar no longer exists.
Clear footerHeight to 0 on unmount so consumers like MessagesList
see the correct value regardless of breakpoint transitions.