feat: Implement a structured API service layer for authentication and user management, replacing the old API utility.

src/services/index.ts

@@ -0,0 +1,21 @@
+// API Client
+export { default as api } from './api';
+export type { ApiResponse, ApiErrorResponse } from './api';
+export { getErrorMessage, getFieldErrors } from './api';
+
+// Auth Service
+export { authService } from './auth.service';
+export type {
+  LoginRequest,
+  AuthUser,
+  AuthResponse,
+  MessageResponse,
+} from './auth.service';
+
+// Users Service
+export { usersService } from './users.service';
+export type {
+  User,
+  UsersListResponse,
+  UserFilters,
+} from './users.service';