Scheduler Timeline Editing

Create and edit timeline events through the shared scheduler dialog with DateTimePicker.

• Adding events from a toolbar button or empty lane clicks
• Editing title, start/end, and resource assignment on existing bars

• Wrap the timeline in DatetimeLocale so pickers share timezone and format.
• Set event_creation=Signal::from(true) on SchedulerTimeline.
• Provide a trigger (button or slot child) that calls the interaction context to open the dialog.
• Bind events as RwSignal<Vec<PlannedEvent>> — saves merge into this collection.