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.