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/_layout.tsx

@@ -2,5 +2,13 @@ import { Stack } from "expo-router";
 import "../../global.css";
 
 export default function RootLayout() {
-  return <Stack screenOptions={{ headerShown: false }} />;
+  return (
+    <Stack screenOptions={{ headerShown: false }}>
+      <Stack.Screen name="(tabs)" />
+      <Stack.Screen name="login" />
+      <Stack.Screen name="register" />
+      <Stack.Screen name="event/[id]" />
+      <Stack.Screen name="note/[id]" />
+    </Stack>
+  );
 }