fix(storefront): complete the cart line-item chain

[roncodes]

Update item in Cart and Remove item from cart addressed /carts/{id}/{line_item_id}. Single braces are not Postman variables, so both requested literal placeholders and answered "Invalid cart item provided to cart!".

Nothing captured a line item id anywhere in the collection either — and it is the cart item id (cart_item_*), not the product id, so {{product_id}} would not have worked as a substitute.

Add Item to Cart now captures {{line_item_id}} from items[0].id of the cart it returns, and both requests use {{cart_id}}/{{line_item_id}}.

Two more things in the same request

• Update item in Cart had no body at all, so the PUT updated nothing even when it reached the endpoint. It now sends {"quantity": 2} — CartController::update() reads quantity, variants, addons and scheduled_at, leaving omitted fields unchanged.
• Its description read "You can update all properties of the Zone." — copy-pasted from the Zones collection into published Storefront documentation. Replaced with what the endpoint actually does.

This is the tail of the chain Add Item to Cart unblocked in fleetbase/fleetbase#598; that one now passes and these two were the only cart requests still failing behind it.

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