fix(admin): OrderEdit display of difference due with refund (#3487)

**What**
- fix `difference_due` in the OE modal to show the correct difference a user has to pay 
- fix Timeline crashing due to returned line item being `undefined `(deleted after OE confirmation)

**Why**
We were taking into account `refunded_amount` when calculating "Difference Due"  and used the following formula: 
`orderEdit.total - paidTotal + refundedTotal`.
The issue here is that  `orderEdit.total` is "overpriced" for the amount of items that are returned/refunded already (i.e. we are charging the user again for the returned items).

The straightforward solution would be to subtract `refunded_total` from the `orderEdit.total` to get the total cost of items the user is actually getting after the edit.

`difference_due = (orderEdit.total - refundedTotal) - (paidTotal - refundedTotal ) =  orderEdit.total - paidTotal`

But if we have partially refunded returns, we would have the same problem of returned items priced into the total.
In that case, general solution should be something like:
`diff_due_display = orderEdit_total_of_items_user_is_getting - paid_total + refunded_total`

Would like to hear what you guys think!

---

FIXES CORE-1246

packages/admin-ui/ui/src/domain/orders/edit/modal.tsx

@@ -336,7 +336,10 @@ function OrderEditModal(props: OrderEditModalProps) {
               currencyCode={currencyCode}
               amountPaid={paidTotal - refundedTotal}
               newTotal={orderEdit.total}
-              differenceDue={orderEdit.total - paidTotal + refundedTotal}
+              differenceDue={
+                // TODO: more correct would be to have => diff_due = orderEdit_total_of_items_user_is_getting - paid_total + refunded_total
+                orderEdit.total - paidTotal // (orderEdit_total - refunded_total) - (paidTotal - refundedTotal)
+              }
             />
           )}