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/services/ApiClient.ts

@@ -0,0 +1,51 @@
+const API_BASE_URL =
+  process.env.EXPO_PUBLIC_API_URL || "http://localhost:3000/api";
+
+type HttpMethod = "GET" | "POST" | "PUT" | "DELETE";
+
+interface RequestOptions {
+  headers?: Record<string, string>;
+  body?: unknown;
+}
+
+async function request<T>(
+  _method: HttpMethod,
+  _endpoint: string,
+  _options?: RequestOptions,
+): Promise<T> {
+  throw new Error("Not implemented");
+}
+
+export const ApiClient = {
+  get: async <T>(
+    endpoint: string,
+    options?: Omit<RequestOptions, "body">,
+  ): Promise<T> => {
+    return request<T>("GET", endpoint, options);
+  },
+
+  post: async <T>(
+    endpoint: string,
+    body?: unknown,
+    options?: RequestOptions,
+  ): Promise<T> => {
+    return request<T>("POST", endpoint, { ...options, body });
+  },
+
+  put: async <T>(
+    endpoint: string,
+    body?: unknown,
+    options?: RequestOptions,
+  ): Promise<T> => {
+    return request<T>("PUT", endpoint, { ...options, body });
+  },
+
+  delete: async <T>(
+    endpoint: string,
+    options?: Omit<RequestOptions, "body">,
+  ): Promise<T> => {
+    return request<T>("DELETE", endpoint, options);
+  },
+};
+
+export { API_BASE_URL };