Scheduler Event Timeline Preferences

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

• Personalization for Gantt-style timelines shared with calendar preferences
• Day preset views where AM/PM vs 24-hour labels matter

• Build SchedulerPreferences via scheduler_preferences_from_signals or a snapshot.
• Pass preferences to SchedulerTimeline.
• Wire SchedulerPreferencesMenu toggles to the same signals for live updates.