Settings > General > Language & region gets "Week starts on" (Monday,
Sunday or Saturday), and Admin > User Defaults a default for everyone who
has not picked one. Monday stays the built-in value, so nothing changes
for anyone who leaves it alone.
Both calendar implementations follow it: CustomDatePicker, used by trip
dates, bookings, costs, to-dos and the admin school holidays on desktop
and phone, and the journey date picker, whose weekday headers were
hardcoded English and now speak the app language. The grid math and the
header row live in one helper (utils/calendarWeek.ts). The day names on
the options come from Intl, so only the label and a hint needed
translating. Vacay keeps its own per-plan week start.
The value is validated like the other regional settings: VALID_VALUES for
admin defaults, the display-settings MCP tool, and the managed key list.
Addresses discussion #2029.
Maurice
7ce31756a3