Replace the AUTO/MONDAY/SUNDAY week-start enum with a sealed type
(Auto + Day(DayOfWeek)) so users can pick any of the seven days as the
first day of the week. The picker now lists "Automatic" plus all seven
localised weekday names; labels come from java.time display names rather
than per-day string resources.
Stored values round-trip by DayOfWeek.name, so the legacy MONDAY/SUNDAY
preferences migrate transparently. Garbage values fall back to Auto.
Closes #3 (Codeberg)
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
