implement frontend skeleton with tab navigation and service layer

- Add tab-based navigation (Chat, Calendar) using Expo-Router
- Create auth screens (login, register) as skeletons
- Add dynamic routes for event detail and note editing
- Implement service layer (ApiClient, AuthService, EventService, ChatService)
- Add Zustand stores (AuthStore, EventsStore) for state management
- Create EventCard and EventConfirmDialog components
- Update CLAUDE.md with new frontend architecture documentation
- Add Zustand and FlashList to technology stack

apps/client/src/app/note/[id].tsx

@@ -0,0 +1,34 @@
+import { View, Text, TextInput, Pressable } from 'react-native';
+import { useLocalSearchParams } from 'expo-router';
+import BaseBackground from '../../components/BaseBackground';
+
+const NoteScreen = () => {
+  const { id } = useLocalSearchParams<{ id: string }>();
+
+  // TODO: Fetch event by id using EventService.getById()
+  // TODO: Display and edit the event's note field
+  // TODO: Auto-save or manual save button
+  // TODO: Save changes -> EventService.update({ note: ... })
+  // TODO: Loading and error states
+  throw new Error('Not implemented');
+
+  return (
+    <BaseBackground>
+      <View className="flex-1 p-4">
+        <Text className="text-2xl mb-4">Note</Text>
+        <Text className="text-gray-500 mb-4">Event ID: {id}</Text>
+        <TextInput
+          placeholder="Write your note here..."
+          multiline
+          className="w-full border rounded p-2 flex-1 mb-4"
+          textAlignVertical="top"
+        />
+        <Pressable className="bg-blue-500 p-3 rounded">
+          <Text className="text-white text-center">Save Note</Text>
+        </Pressable>
+      </View>
+    </BaseBackground>
+  );
+};
+
+export default NoteScreen;