diag: Probe actual VStack size after .frame is applied

Adds an unobtrusive GeometryReader background that logs the VStack's
real rendered size. Pinpoints whether the published 1.5.4 binary's
visible empty-band issue is caused by .frame not taking effect
(VStack size != popoverHeight) or by the menu-bar panel itself
not resizing (VStack size matches but panel is bigger). Local
Release builds render correctly, so the bug only reproduces in
the CI-built signed/notarized binary — this probe will let us tell
which from the released-build log.