Add Simplified Chinese as a system language

The installer asks for the language before the keyboard, and
Setup > Language (omarchy language en|zh) switches it later. Under
zh_CN the menu reads its labels from omarchy-menu.zh-cn.jsonc, the
shell from I18nZh.js through I18n.tr, and notifications are worded
by their sender through omarchy-text, which reads the same table.
User text is never translated.

bin/omarchy-theme-set

@@ -190,7 +190,7 @@ set_theme_background() {
 
   if [[ -z $CHOSEN_THEME_BACKGROUND || ! -f $CHOSEN_THEME_BACKGROUND ]]; then
     if ! choose_theme_background; then
-      omarchy-notification-send "No background was found for theme" -t 2000
+      omarchy-notification-send "$(omarchy-text "No background was found for theme")" -t 2000
       shell_ipc shell applyTheme "$colors_payload" "$shell_payload" || true
       return
     fi