chore(dist): add Linux desktop integration files for installation

Desktop entry, AppStream metainfo, D-Bus service and systemd user unit
templates plus icons, adapted from ZIG/dist/linux and used by
`just install`.

dist/linux/systemd.service.in

@@ -0,0 +1,14 @@
+[Unit]
+Description=@NAME@
+After=graphical-session.target
+After=dbus.socket
+Requires=dbus.socket
+
+[Service]
+Type=notify-reload
+ReloadSignal=SIGUSR2
+BusName=@APPID@
+ExecStart=@GHOSTTY@ --gtk-single-instance=true --initial-window=false
+
+[Install]
+WantedBy=graphical-session.target