Make clock's alt format include the ISO week number

Set the default formatAlt to "dd MMMM 'W'ww yyyy" (was "dd MMMM yyyy")
in shell-defaults.json, the bundled settings-panel defaults, and the
form placeholder text — clock's own widget fallback already used this
spec but the shipped config overrode it.

Qt.formatDateTime doesn't recognize `ww` and renders it literally
(`20 May Www 2026`), so add an `isoWeek` helper to clock.qml that
pre-substitutes `ww` with a Qt-quoted literal of the ISO 8601 week
number before handing the format off to Qt. Users get `20 May W21 2026`
without anyone needing to know about the substitution.

shell/plugins/settings/SettingsPanel.qml

@@ -132,7 +132,7 @@ Item {
       layout: {
         left: [{ id: "omarchy" }, { id: "workspaces" }],
         center: [
-          { id: "clock", format: "dddd HH:mm", formatAlt: "dd MMMM yyyy", verticalFormat: "HH\n\u2014\nmm" },
+          { id: "clock", format: "dddd HH:mm", formatAlt: "dd MMMM 'W'ww yyyy", verticalFormat: "HH\n\u2014\nmm" },
           { id: "weather" }, { id: "indicators", items: [ "dnd", "nightlight", "stayAwake", "screenrecording", "dictation" ] }, { id: "systemUpdate" }
         ],
         right: [
@@ -1205,7 +1205,7 @@ Item {
       }
       ClockField {
         fieldKey: "formatAlt"
-        text: clockForm.entry.formatAlt || "dd MMMM yyyy"
+        text: clockForm.entry.formatAlt || "dd MMMM 'W'ww yyyy"
       }
 
       Text {