Scheduler Event Calendar Preferences

Let users toggle weekend visibility, 12/24-hour time, and week start day on SchedulerCalendar.

• Personalization menus in scheduling apps
• Persisting display settings alongside user profiles

• Build SchedulerPreferences from signals or a snapshot via scheduler_preferences_from_signals.
• Pass preferences to SchedulerCalendar.
• Optionally listen to on_preferences_change to persist toggles.