Replace xdg-decoration (needs xdg_toplevel, GTK3 doesn't expose it)
with org_kde_kwin_server_decoration (works with wl_surface which
GDK provides). On KDE Wayland: binds the manager from the registry,
creates a per-surface decoration object, requests mode=None (0).
Falls back gracefully on GNOME (protocol not present, GTK's
set_decorated(FALSE) already works). X11 fallback via
gdk_window_set_decorations(0) unchanged.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
