feat: add EditEventScreen with calendar and chat mode support

Add a unified event editor that works in two modes:
- Calendar mode: Create/edit events directly via EventService API
- Chat mode: Edit AI-proposed events before confirming them

The chat mode allows users to modify proposed events (title, time,
recurrence) and persists changes both locally and to the server.

New components: DateTimePicker, ScrollableDropdown, useDropdownPosition
New API: PUT /api/chat/messages/:messageId/proposal

packages/shared/src/utils/index.ts

@@ -1,3 +1,3 @@
 export * from "./dateHelpers";
-export * from "./rruleHelpers";
 export * from "./formatters";
+export * from "./rruleHelpers";