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/server/src/routes/chat.routes.ts

@@ -19,6 +19,9 @@ export function createChatRoutes(chatController: ChatController): Router {
   router.get("/conversations/:id", (req, res) =>
     chatController.getConversation(req, res),
   );
+  router.put("/messages/:messageId/proposal", (req, res) =>
+    chatController.updateProposalEvent(req, res),
+  );
 
   return router;
 }