add Zustand auth and cart stores with secure token persistence

[wumibals]

Sets up global state management for the mobile app using Zustand, covering the two most critical slices — authentication and the cart.

useAuthStore holds the user object, token, and login/logout actions. The token is persisted to expo-secure-store so sessions survive app restarts, and the store hydrates it automatically on load. Logging out clears both in-memory state and the secure storage entry.

useCartStore manages the array of matched food items with add, remove, and clear actions.

Acceptance criteria

• ✅ Auth token persists across restarts via expo-secure-store
• ✅ Store hydrates the token correctly on app load
• ✅ Logout clears state and removes token from secure storage
• ✅ Cart supports add, remove, and clear

closes #139