feat(island): date and time format settings

The clock pill hard-coded "ddd dd/MM" and "HH:mm", and the calendar its
day headings. A new Date and time page in the Island bar group picks a
24- or 12-hour clock and one of 22 date formats, grouped by order (day,
month or year first) and each shown as today's date. The calendar's
headings and editor dates keep spelling the month out and follow the
chosen order; agenda times and the hour picker go compact on the
12-hour clock ("9:30p", "1p"), where width is short. A preview mocks the
pill and panel with the unsaved choice.

The formats live in Cal.js (DATE_GROUPS), which Prefs imports to
validate island.dateFormat. Event times still store and show as 24-hour
HH:MM, since that is what calctl speaks, but normTime now also accepts
am and pm ("9.30pm", "12am"), rejecting an hour outside 1 to 12 with one.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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