An isolated parallel-plate capacitor with plate area \(A\) carries a constant charge of magnitude \(Q_0\) on each plate. A dielectric slab of dielectric constant \(\kappa > 1\) initially fills the space between the plates. A closed Gaussian pillbox has one face inside the positive plate and the opposing face in the region between the plates within the dielectric, with side walls perpendicular to the plates. When the dielectric slab is completely withdrawn from the capacitor, how do the net electric flux through the Gaussian surface and the total electrostatic potential energy stored in the capacitor change?