UI: convert LXDE/GNOME/KDE submenus to _menu style (XFCE parity)

Replace _radiolist with the _menu pattern used by xfce_menu:
while-true loop, fixed numbered items array, Back option that
breaks the loop, and direct case dispatch to the unchanged
installers. Navigation is now consistent across all four desktop
submenus (select an option, return to the parent menu, re-enter
without re-triggering a one-shot selection).

- bash -n OK, shellcheck 0 warnings, BATS 36/0
- _radiolist helper kept in utils.sh for compatibility (no callers)

modules/desktops/desktop_gnome.sh

@@ -5,19 +5,23 @@
 # License GPL v3
 
 gnome_menu() {
-    local choice
-    choice=$(_radiolist "GNOME" \
-        "Select an option:" $TUI_ALTO $TUI_ANCHO $TUI_ALTO_LISTA \
-        "1" "GNOME (Full Desktop)" OFF \
-        "2" "GNOME Core (Minimal Install)" OFF \
-        "3" "Back" OFF)
-    [ -z "$choice" ] && return 0
-    clear
-    case "$(echo "$choice" | tr -d '"')" in
-    1) _install_gnome_full ;;
-    2) _install_gnome_core ;;
-    3) return 0 ;;
-    esac
+    while true; do
+        local -a gn_items=()
+        gn_items+=("1" "GNOME (Full Desktop)")
+        gn_items+=("2" "GNOME Core (Minimal Install)")
+        gn_items+=("3" "Back")
+        local choice
+        choice=$(_menu "GNOME" \
+            "Select an option:" $TUI_ALTO $TUI_ANCHO $TUI_ALTO_LISTA \
+            "${gn_items[@]}")
+        [ -z "$choice" ] && break
+        clear
+        case "$choice" in
+        1) _install_gnome_full ;;
+        2) _install_gnome_core ;;
+        3) break ;;
+        esac
+    done
 }
 
 _install_gnome_full() {