fix duplicate sidebar items after rescheduling calendar event (notes should follow updated start datetime)

[ComputelessComputer]

problem

• in the left sidebar, if you click a future calendar event and start typing, it creates a note attached to that event.
• if you later reschedule that event in the calendar (change start time), the sidebar shows two items with the same event name:
  • one at the old time
  • one at the new time
• this creates dupes and the sidebar item does not reflect the synced start datetime.

expected

• rescheduling an event should not create a second sidebar item.
• the existing sidebar item should update to the event’s latest synced start_datetime.
• the note remains attached to the same calendar event after reschedule.

actual

• after rescheduling, two sidebar entries appear for the “same” event name (old + new time).

repro steps

1. open app
2. in left sidebar, click a future calendar event
3. start typing to create a note attached to the event
4. open calendar app and reschedule that event to a different start time (same event, new datetime)
5. return to app / wait for sync
6. observe duplicate sidebar items (old time + new time)

notes / hints

• likely using start_datetime (or a derived key) as part of the sidebar item identity.
• should key notes/sidebar items by a stable event identifier (provider event id / iCal UID) and treat datetime changes as updates, not inserts.

acceptance criteria

• rescheduling a calendar event updates the existing sidebar item’s datetime and does not create duplicates.
• note stays attached to the event across reschedule.
• works across at least: reschedule forward/backward in time, and multiple reschedules.