Feat: Gregorian Calendar watch face

[moritzgloeckl]

New Gregorian Calendar face. Supports scrolling back & forth and holidays.

European/Gregorian Calendar Watch Face

This watch face displays the current date, weekday, and highlights public holidays for the selected country.

Setup:

• Holidays are configured in calendar_face.c via the holidays[] array.
  To change the country or add/remove holidays, edit the array and use macros from calendar_face_holidays.h.
  Both fixed-date and dynamic (Easter-based) holidays are supported.

Usage:

• By default, the watch face shows today's date and highlights if it is a holiday.
• Hold ALARM to enter scrolling mode (shows today as starting point).
• Press ALARM to scroll forward (day/month/year depending on mode).
• Press LIGHT to scroll backward.
• Hold ALARM or LIGHT for fast scrolling (7 days or 3 months/years).
• Press MODE to switch between day, month, and year scrolling modes.
• Hold MODE to exit scrolling mode and return to the default view.
• Timeout or long MODE press returns to the first watch face.
• The bell indicator shows when the selected date is a public holiday.

To customize holidays, use the macros in calendar_face_holidays.h and update the holidays[] array in calendar_face.c.