feat(anaconda): add desktop environment selection spoke for FWS

Implement a new Anaconda spoke that allows users to select their preferred desktop environment (CLI, GNOME, KDE Plasma, Hyprland, or XFCE) during installation.

The spoke:
- Displays radio buttons for five desktop choices with CLI as the default
- Persists the selection to /tmp/fws-desktop for the kickstart %post script
- Supports revisiting the spoke and resuming previous selections
- Includes a GTK UI definition file (glade) with translatable labels
- Integrates as a SoftwareCategory spoke in the Anaconda installer

The implementation leverages Anaconda's runtime spoke discovery mechanism via the airootfs overlay, avoiding the need to rebuild the anaconda package.

configs/releng/airootfs/usr/share/anaconda/interactive-defaults.ks

@@ -216,6 +216,8 @@ case "$DESK" in
     hyprland) PKGS="hyprland kitty wofi waybar sddm";                  DM="sddm" ;;
     cli|*)    PKGS="" ;;
 esac
+# Navigateur de base ajouté DÈS QU'un bureau est choisi (jamais en CLI).
+[ -n "$PKGS" ] && PKGS="$PKGS firefox"
 if [ -n "$PKGS" ]; then
     pacman-key --init            &>/dev/null || true
     pacman-key --populate archlinux &>/dev/null || true