Scheduler Calendar Editing

Create and edit calendar events through the shared dialog with DateTimePicker.

• Adding events from empty timed slots or a toolbar button
• Editing title, start/end, and resource assignment on existing chips

• Wrap SchedulerCalendar in DatetimeLocale for picker timezone and format.
• Set event_creation=Signal::from(true) to enable click-to-create on empty slots.
• Provide SchedulerEditingTools slot content that opens the dialog via use_scheduler_interaction.
• Bind events: RwSignal<Vec<PlannedEvent>> for the saved collection.