[Feature Request] Support adding items to the shopping basket using `productId`

### UC2: Add selected item to the basket — ideal for Home Assistant Voice Preview Edition ###

I’ve identified a working API endpoint for adding items to the cart:

```
POST https://www.rohlik.cz/api/v1/cart/item
Payload: { amount: 1, productId: 1461047, source: "true:Search Results" }
```