XFCE on SDDM; menu defaults to none for DE and TWM

- XFCE module installs/enables sddm instead of lightdm; one greeter across
  every session choice, no lightdm left anywhere.
- Interactive menus now default to 'none' for both DE and TWM (press Enter
  for no DE / no TWM). none+none is still refused by the existing guard.
- README and session.sh comments updated to match.

README.md

@@ -61,7 +61,7 @@ Each module pulls in `lib/` by relative path, so it works both ways.
 |---|---|---|
 | `gnome` | `gnome-desktop`, `gnome-desktop-environment`, `workstation-product-environment` | gdm |
 | `plasma` | `kde-desktop`, `kde-desktop-environment` | sddm |
-| `xfce` | `xfce-desktop`, `xfce-desktop-environment` | lightdm |
+| `xfce` | `xfce-desktop`, `xfce-desktop-environment` | sddm |
 
 ### Tiling window managers
 
@@ -78,7 +78,7 @@ compositors — do not expect the same config story from it.
 ## Display manager policy
 
 * A **DE module** leaves the DE's own greeter alone (GNOME→gdm, Plasma→sddm,
-  XFCE→lightdm). It never fights a desktop over its own login screen.
+  XFCE→sddm). It never fights a desktop over its own login screen.
 * A **TWM module** enforces **SDDM**, because it is the only greeter that lists
   Wayland compositors *and* X11 sessions in one menu.
 * Either way `graphical.target` becomes the default.

lib/session.sh

@@ -3,7 +3,7 @@
 # lib/session.sh — display manager and graphical session wiring
 #
 # DM policy:
-#   * A DE module leaves its native DM alone (GNOME→gdm, Plasma→sddm, XFCE→lightdm).
+#   * A DE module leaves its native DM alone (GNOME→gdm, Plasma→sddm, XFCE→sddm).
 #   * A TWM module enforces SDDM, because SDDM is the only DM that presents both
 #     Wayland compositors and X11 sessions in one menu. One greeter, no surprises.
 #------------------------------------------

modules/de/xfce.sh

@@ -24,11 +24,11 @@ fi
 # provider for chadwm.
 install_packages xorg-x11-server-Xorg xorg-x11-xinit
 
-# XFCE's native greeter is lightdm
+# XFCE runs fine under SDDM too — unify on SDDM so every session has one greeter
-if ! pkg_installed lightdm; then
+if ! pkg_installed sddm; then
-    install_packages lightdm
+    install_packages sddm
 fi
-switch_display_manager lightdm
+switch_display_manager sddm
 
 install_nerd_font JetBrainsMono
 install_packages alacritty dconf

setup.sh

@@ -210,10 +210,10 @@ if [[ -z "$DE" ]]; then
     printf "\n"
     echo "  1) GNOME     (gdm)"
     echo "  2) Plasma    (sddm)"
-    echo "  3) XFCE      (lightdm)"
+    echo "  3) XFCE      (sddm)"
     echo "  4) None      (TWM only — SDDM will be installed by the TWM module)"
     printf "\n"
-    choice=$(ask_choice "Select desktop environment [1/2/3/4]" "1" 1 2 3 4)
+    choice=$(ask_choice "Select desktop environment [1/2/3/4]" "4" 1 2 3 4)
     case "$choice" in
         1) DE="gnome" ;;
         2) DE="plasma" ;;
@@ -241,7 +241,7 @@ if [[ -z "$TWM" ]]; then
     echo "  4) chadwm     (X11, dwm fork, built from source)"
     echo "  5) None"
     printf "\n"
-    choice=$(ask_choice "Select tiling window manager [1/2/3/4/5]" "1" 1 2 3 4 5)
+    choice=$(ask_choice "Select tiling window manager [1/2/3/4/5]" "5" 1 2 3 4 5)
     case "$choice" in
         1) TWM="niri" ;;
         2) TWM="hyprland" ;;