Backend Cart Model

Mirrors the Medusa Cart entity returned by /store/carts/{id}: id, region, currency, line items, totals (total/subtotal/itemTotal/shipping/discount/tax), email, shipping & billing addresses.

1. Cart.fromJson maps the Medusa payload into a typed Cart.
2. CartItem.fromJson maps a single line-item including thumbnail + variant info.
3. CheckoutScreen consumes the Cart to render line items, totals and the address forms.

• currencyCode defaults to 'eur' when missing — Medusa always sends it, but the fallback protects against legacy fixtures.
• shippingAddress / billingAddress are nullable until the user fills them in; CheckoutScreen treats null as 'collect address first'.
• discountTotal and taxTotal are computed server-side; local recomputation drifts because of region-specific tax rules.

• lib/src/features/shared/cart/models/cart.dart:4 // Cart
• lib/src/features/shared/cart/models/cart_item.dart // CartItem
• lib/src/features/shared/checkout/models/address.dart // Address

The backend cart is the source of truth for prices — never trust the local cart's totalPrice when displaying anything monetary post-checkout-start.