CmdPal: Add calendar page to time/date dock band clock item

Clicking the time/date item in the CmdPal dock now opens a calendar
page showing the current month with today's date highlighted in bold.

- Add CalendarPage (ContentPage) that renders a markdown table calendar
  respecting the locale's first day of week and abbreviated day names
- Wire NowDockBand.Command to CalendarPage instead of NoOpCommand
- Add timedate_calendar_page_name resource string
- Add CalendarPageTests covering structure, today highlighting, and dock wiring

Co-authored-by: niels9001 <[email protected]>
Agent-Logs-Url: https://github.com/microsoft/PowerToys/sessions/1d52e678-b258-441c-a409-1de12a42cb2a