feat: add Zustand stores and custom hooks

frontend-mobile/store/authStore.ts

@@ -0,0 +1,38 @@
+import { create } from 'zustand';
+import { User } from '../types';
+import * as authService from '../services/auth';
+
+interface AuthState {
+  user: User | null;
+  isLoading: boolean;
+  isAuthenticated: boolean;
+  login: (username: string, password: string) => Promise<void>;
+  logout: () => Promise<void>;
+  checkAuth: () => Promise<void>;
+}
+
+export const useAuthStore = create<AuthState>((set) => ({
+  user: null,
+  isLoading: true,
+  isAuthenticated: false,
+
+  login: async (username: string, password: string) => {
+    const { user } = await authService.login(username, password);
+    set({ user, isAuthenticated: true });
+  },
+
+  logout: async () => {
+    await authService.logout();
+    set({ user: null, isAuthenticated: false });
+  },
+
+  checkAuth: async () => {
+    set({ isLoading: true });
+    const user = await authService.checkAuth() || await authService.getStoredUser();
+    set({ 
+      user, 
+      isAuthenticated: !!user, 
+      isLoading: false 
+    });
+  },
+}));