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/app/_layout.tsx

@@ -7,8 +7,9 @@ export default function RootLayout() {
       <Stack.Screen name="(tabs)" />
       <Stack.Screen name="login" />
       <Stack.Screen name="register" />
-      <Stack.Screen name="event/[id]" />
-      <Stack.Screen name="note/[id]" />
+      <Stack.Screen name="editEvent" />
+      {/* <Stack.Screen name="event/[id]" /> */}
+      {/* <Stack.Screen name="note/[id]" /> */}
     </Stack>
   );
 }