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.

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" ;;