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/flatpak-user-firstboot

@@ -0,0 +1,24 @@
+#!/usr/bin/env bash
+# Install the per-user Flatpaks listed in /usr/share/flatpak/flatpaks.list on
+# the first login of each user. Run as a systemd --user oneshot unit.
+set -euo pipefail
+
+LIST="/usr/share/flatpak/flatpaks.list"
+MARKER="${HOME}/.config/flatpak-user-firstboot.done"
+
+[[ -f "${LIST}" ]] || exit 0
+
+# Make Flathub available for the current user.
+flatpak remote-add --user --if-not-exists flathub \
+  https://flathub.org/repo/flathub.flatpakrepo
+
+mapfile -t apps < <(
+  sed -e 's/#.*//' -e 's/[[:space:]]//g' "${LIST}" | grep -v '^$' || true
+)
+
+if [[ ${#apps[@]} -gt 0 ]]; then
+  flatpak install --user --noninteractive --assumeyes "${apps[@]}"
+fi
+
+install -dm0755 "$(dirname "${MARKER}")"
+touch "${MARKER}"