getCartItems method
Retrieves a stream of the user's cart items from Firestore.
Returns: A Stream<List<CartItem>> containing the user's cart items.
Implementation
Stream<List<CartItem>> getCartItems() {
return _firestore
.collection('carts')
.doc(userId)
.collection('items')
.snapshots()
.map((snapshot) =>
snapshot.docs.map((doc) => CartItem.fromMap(doc.data())).toList());
}