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/controllers/ChatController.ts

@@ -150,4 +150,33 @@ export class ChatController {
       }
     }
   }
+
+  async updateProposalEvent(
+    req: AuthenticatedRequest,
+    res: Response,
+  ): Promise<void> {
+    try {
+      const { messageId } = req.params;
+      const { proposalId, event } = req.body as {
+        proposalId: string;
+        event: CreateEventDTO;
+      };
+      const message = await this.chatService.updateProposalEvent(
+        messageId,
+        proposalId,
+        event,
+      );
+      if (message) {
+        res.json(message);
+      } else {
+        res.status(404).json({ error: "Message or proposal not found" });
+      }
+    } catch (error) {
+      log.error(
+        { error, messageId: req.params.messageId },
+        "Error updating proposal event",
+      );
+      res.status(500).json({ error: "Failed to update proposal event" });
+    }
+  }
 }