Enabling widget.gtk.rounded-bottom-corners.enabled by default

[cyrneko]

Hi there!

This is a rather small idea, but keeping with the rounded theme that Zen is generally already going for, it may be appropriate to set widget.gtk.rounded-bottom-corners.enabled to true by default to allow the bottom corners to be rounded according to user preferences and/or their desktop too

at least here on a fresh GNOME 46 install, it looks great without any artifacts :P