function cartReducer(state, action) {
    switch (action.type) {
        case 'cart/addItem':
            return {
                ...state,
                items: [...state.items, action.payload]
            };
        case 'cart/removeItem':
            return {
                ...state,
                items: state.items.filter(item => item.id !== action.payload)
            };
        default:
            return state;
    }
}