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_kde.sh

@@ -6,19 +6,23 @@
 # License GPL v3
 
 kde_menu() {
-    local choice
-    choice=$(_radiolist "KDE Plasma" \
-        "Select an option:" $TUI_ALTO $TUI_ANCHO $TUI_ALTO_LISTA \
-        "1" "KDE Plasma (Full Desktop)" OFF \
-        "2" "KDE Plasma Desktop (Minimal)" OFF \
-        "3" "Back" OFF)
-    [ -z "$choice" ] && return 0
-    clear
-    case "$(echo "$choice" | tr -d '"')" in
-    1) _install_kde_full ;;
-    2) _install_kde_minimal ;;
-    3) return 0 ;;
-    esac
+    while true; do
+        local -a kd_items=()
+        kd_items+=("1" "KDE Plasma (Full Desktop)")
+        kd_items+=("2" "KDE Plasma Desktop (Minimal)")
+        kd_items+=("3" "Back")
+        local choice
+        choice=$(_menu "KDE Plasma" \
+            "Select an option:" $TUI_ALTO $TUI_ANCHO $TUI_ALTO_LISTA \
+            "${kd_items[@]}")
+        [ -z "$choice" ] && break
+        clear
+        case "$choice" in
+        1) _install_kde_full ;;
+        2) _install_kde_minimal ;;
+        3) break ;;
+        esac
+    done
 }
 
 _install_kde_full() {