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/models/CalendarEvent.ts

@@ -26,7 +26,6 @@ export interface CreateEventDTO {
   startTime: Date;
   endTime: Date;
   note?: string;
-  isRecurring?: boolean;
   recurrenceRule?: string;
   exceptionDates?: string[]; // For display in proposals
 }
@@ -37,7 +36,6 @@ export interface UpdateEventDTO {
   startTime?: Date;
   endTime?: Date;
   note?: string;
-  isRecurring?: boolean;
   recurrenceRule?: string;
   exceptionDates?: string[];
 }