fix(checkout): take a repeated capture as a successful payment

The CMS answers a capture of an order already paid with this PayPal order
with { success: true, alreadyCaptured: true } instead of the order. Step 3
took that answer for a capture without authorisation and showed a payment
error. It now reloads the paid order (keeping the shown one if the reload
fails) and continues as after a first capture: confirmation, the
checkout-payment-completed event, and the redirect to the order's result
page. capturePayment is typed as Order | AlreadyCaptured, and the pure
isAlreadyCaptured (utils/captureResponse.ts) tells them apart.

Refs libretech/mp#71

composables/useShopApi.ts

@@ -9,6 +9,7 @@ import type {
 	PaymentMethod,
 	ApiResponse
 } from "~/types";
+import type { AlreadyCaptured } from "~/utils/captureResponse";
 
 /**
  * Shop API composable.
@@ -116,8 +117,9 @@ export function useShopApi() {
 			});
 		},
 
-		async capturePayment(uuid: string, paypalOrderId: string): Promise<Order> {
-			return await $fetch<Order>(`/api/orders/${uuid}/capture/${paypalOrderId}`, {
+		/** The paid order, or { success: true, alreadyCaptured: true } when an earlier capture of this PayPal order went through. */
+		async capturePayment(uuid: string, paypalOrderId: string): Promise<Order | AlreadyCaptured> {
+			return await $fetch<Order | AlreadyCaptured>(`/api/orders/${uuid}/capture/${paypalOrderId}`, {
 				method: "POST"
 			});
 		},