A smoother add-to-cart and Buy now experience

Shopping from the video player is clearer and more reliable:

• An "Added to cart" message confirms every add-to-cart.
• The cart button shows the shopper's current item count as soon as the player opens, and is now available on desktop too.
• If an item can't be added (for example it's sold out), the shopper sees it right away instead of a false confirmation.
• Product buttons are stacked full-width, so long translated labels like "In den Warenkorb legen" always fit.