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/bar/widgets/clock.qml

@@ -9,10 +9,31 @@ BarWidget {
 
   property bool alt: false
 
+  // Qt.formatDateTime doesn't recognize `ww` (ISO week number). Pre-substitute
+  // it as a Qt-quoted literal before formatting so users can write the natural
+  // `'W'ww yyyy` and get `W21 2026` rather than `Www 2026`.
+  function isoWeek(date) {
+    var d = new Date(Date.UTC(date.getFullYear(), date.getMonth(), date.getDate()))
+    var day = d.getUTCDay() || 7
+    d.setUTCDate(d.getUTCDate() + 4 - day)
+    var yearStart = new Date(Date.UTC(d.getUTCFullYear(), 0, 1))
+    return Math.ceil(((d - yearStart) / 86400000 + 1) / 7)
+  }
+
+  function format(date, fmt) {
+    var f = String(fmt || "")
+    if (f.indexOf("ww") !== -1) {
+      var w = isoWeek(date)
+      var ww = w < 10 ? "0" + w : String(w)
+      f = f.replace(/ww/g, "'" + ww + "'")
+    }
+    return Qt.formatDateTime(date, f)
+  }
+
   function label() {
-    if (alt) return Qt.formatDateTime(clock.date, String(setting("formatAlt", "dd MMMM 'W'ww yyyy")))
-    if (bar && bar.vertical) return Qt.formatDateTime(clock.date, String(setting("verticalFormat", "HH\n—\nmm")))
-    return Qt.formatDateTime(clock.date, String(setting("format", "dddd HH:mm")))
+    if (alt) return format(clock.date, setting("formatAlt", "dd MMMM 'W'ww yyyy"))
+    if (bar && bar.vertical) return format(clock.date, setting("verticalFormat", "HH\n—\nmm"))
+    return format(clock.date, setting("format", "dddd HH:mm"))
   }
 
   implicitWidth: button.implicitWidth