fedora-remote: minimal headless GNOME Remote Desktop host with Kanidm

New image built from upstream base-atomic (no desktop), adding only gnome-shell
+ gdm + gnome-remote-desktop + pipewire/portal + foot + Flatpak + Kanidm.
recommends=false to stay lean. Configures GNOME Remote Desktop headless remote
login on first boot (TLS + credentials + enable). Firefox preinstalled as a
per-user Flatpak. Add it to the CI matrix and prune both images.

images/fedora-remote/files/grd-firstboot.service

@@ -0,0 +1,15 @@
+[Unit]
+Description=Configure GNOME Remote Desktop on first boot
+Documentation=https://github.com/GNOME/gnome-remote-desktop/blob/main/docs/configuration.md
+After=network-online.target gnome-remote-desktop.service
+Wants=network-online.target
+ConditionPathExists=!/var/lib/gnome-remote-desktop/.configured
+
+[Service]
+Type=oneshot
+EnvironmentFile=-/etc/gnome-remote-desktop/rdp.env
+ExecStart=/usr/libexec/grd-firstboot
+RemainAfterExit=yes
+
+[Install]
+WantedBy=multi-user.target