- Picking a year or month in the calendar's header only moves `DisplayDate`, but `OnCalendarAccepted` read `SelectedDate`, so accepting without clicking a day kept the old date. It now takes the year and month from `DisplayDate` and the selected day, clamped to the month's last day.
- Add headless tests for clicking a day, a day of the next month, and changing only the year or month.

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