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

apps/client/src/services/ChatService.ts

@@ -85,4 +85,15 @@ export const ChatService = {
 
     return ApiClient.get<ChatMessage[]>(url);
   },
+
+  updateProposalEvent: async (
+    messageId: string,
+    proposalId: string,
+    event: CreateEventDTO,
+  ): Promise<ChatMessage> => {
+    return ApiClient.put<ChatMessage>(`/chat/messages/${messageId}/proposal`, {
+      proposalId,
+      event,
+    });
+  },
 };