packaging: depend on the gtk4 GStreamer sink used by the GUI preview

[jordanful]

What

The GUI package doesn't declare the plugin that provides gtk4paintablesink, so the
live preview is broken on a clean install.

Symptom

Fresh install of the released 0.1.4 Arch packages, then opening the preview:

failed to find element factory with name 'gtk4paintablesink'

Why a build doesn't catch it

Preview resolves the sink at runtime (gui/src/preview.rs:150):

let sink = gst::ElementFactory::make("gtk4paintablesink").build()?;

so the package builds, installs and launches fine — only showing the preview fails.
Nothing at compile or package time references the plugin.

Cause

gtk4paintablesink ships in gst-plugins-rs, which distros package separately from
gst-plugins-{base,good}:

distro	package	provides
Arch	gst-plugin-gtk4 (extra)	/usr/lib/gstreamer-1.0/libgstgtk4.so
Debian/Ubuntu	gstreamer1.0-gtk4	libgstgtk4.so

packaging/nfpm-gui.yaml declares gtk4 and libadwaita, but not this.

Verification

Built the GUI package both ways with nfpm 2.47 and read .PKGINFO:

package	Depends On
released openeffects-0.1.4-1-x86_64.pkg.tar.zst	openeffectsd gtk4 libadwaita
local rebuild, unpatched	openeffectsd gtk4 libadwaita (faithful repro)
local rebuild, patched	openeffectsd gtk4 libadwaita gst-plugin-gtk4

Then end-to-end in a clean archlinux:latest container, installing openeffectsd plus
the GUI package:

• unpatched: error: package 'gst-plugin-gtk4' was not found; gtk4paintablesink MISSING
• patched: pacman resolves and pulls gst-plugin-gtk4 0.15.3-1 from extra; gtk4paintablesink OK

Note on the rpm override

I left overrides.rpm untouched on purpose. I couldn't find a Fedora package shipping this
plugin — gstreamer1-plugins-rs and gstreamer1-plugin-gtk4 don't resolve in Fedora's
package index.