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.

shell/Ui/SpeedTestOverlay.qml

@@ -23,7 +23,7 @@ PanelWindow {
   property string unit: "Mbps"
   property string title: ""
   property string layerNamespace: "omarchy-speed-test"
-  property string runAgainTooltip: "Measure again"
+  property string runAgainTooltip: I18n.tr("Measure again")
   property real leftValue: 0
   property real rightValue: 0
   property bool leftLive: false
@@ -165,7 +165,7 @@ PanelWindow {
         // Centered on the dial pair. Fades rather than unmounts while a run
         // is in flight, so the cluster never shifts.
         Button {
-          text: "Run Again"
+          text: I18n.tr("Run Again")
           tooltipText: root.runAgainTooltip
           bordered: true
           enabled: !root.running