qt: add a resizable margin to the application in CSD mode when extended frames aren't available

fix #28906 on platform that don't support extended frames

this fix is mainly designed for compositor that don't have extended frames such as X11 with no _GTK_FRAME_EXTENTS support or the dummy compositor.

This is not intended to replace !6510, it doesn't even look particularly good on windows 11 with the rounded corners

This patch adds a thin frame around the content, this is similar to what Firefox or Chrome do in similar scenarios. An alternative would have been to update the safe-area to the resize margin, but the scrollbar tend to stay on the edge of the frame regardless of the controls padding, so this doesn't solve the issue