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/shell-defaults.json

@@ -21,7 +21,7 @@
         {
           "id": "clock",
           "format": "dddd HH:mm",
-          "formatAlt": "dd MMMM yyyy",
+          "formatAlt": "dd MMMM 'W'ww yyyy",
           "verticalFormat": "HH\n\u2014\nmm"
         },
         {