fix(menu): theme and background submenus return to parent instead of exiting

The menu navigation contract: a submenu invoked directly via keybinding
(BACK_TO_EXIT=true, set by go_to_menu when nomarchy-menu is launched
with a target argument) should `exit 0` after the user's action; a
submenu invoked from a parent menu (BACK_TO_EXIT=false) should call
`back_to <parent>` to return where the user came from. back_to() honors
both modes.

Three submenus violated the contract:

- show_theme_menu and show_background_menu shell out to walker's
  Elephant plugin and don't call back_to. After picking a theme or
  wallpaper from Main -> Style -> Theme, the script exits silently
  instead of returning to Style; the user has to relaunch the menu
  from scratch to change anything else.

- show_hardware_menu's cancel branch called show_trigger_menu directly
  instead of back_to show_trigger_menu, which would have bounced a
  direct-keybinding caller into Trigger instead of exiting cleanly.

Adds the missing back_to call to the two walker-backed submenus
(parented to show_style_menu) and converts the hardware cancel branch
to back_to. The 16 other show_*_menu functions already conform.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

features/scripts/utils/nomarchy-menu

@@ -161,7 +161,7 @@ show_toggle_menu() {
 show_hardware_menu() {
   case $(menu "Toggle" "  Hybrid GPU") in
   *"Hybrid GPU"*) present_terminal nomarchy-toggle-hybrid-gpu ;;
-  *) show_trigger_menu ;;
+  *) back_to show_trigger_menu ;;
   esac
 }
 
@@ -179,10 +179,12 @@ show_style_menu() {
 
 show_theme_menu() {
   nomarchy-launch-walker -m menus:nomarchythemes --width 800 --minheight 400
+  back_to show_style_menu
 }
 
 show_background_menu() {
   nomarchy-launch-walker -m menus:nomarchyBackgroundSelector --width 800 --minheight 400
+  back_to show_style_menu
 }
 
 show_font_menu() {