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

tests/unit/captureResponse.test.ts

@@ -0,0 +1,21 @@
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import { isAlreadyCaptured } from "../../utils/captureResponse.ts";
+
+test("recognises the CMS's answer to a repeated capture", () => {
+	assert.equal(isAlreadyCaptured({ success: true, alreadyCaptured: true }), true);
+});
+
+test("takes the paid order of a first capture for no repeated capture", () => {
+	const order = { uuid: "11111111-2222-4333-8444-555555555555", total: 22.85, paymentAuthorised: true, email: "erika@example.org" };
+
+	assert.equal(isAlreadyCaptured(order), false);
+	assert.equal(isAlreadyCaptured({ ...order, alreadyCaptured: false }), false);
+});
+
+test("takes nothing but true for both fields as a repeated capture", () => {
+	assert.equal(isAlreadyCaptured({ alreadyCaptured: true }), false);
+	assert.equal(isAlreadyCaptured({ success: true, alreadyCaptured: "true" }), false);
+	assert.equal(isAlreadyCaptured({ success: false, alreadyCaptured: true }), false);
+	for (const response of [undefined, null, "alreadyCaptured", 1, [true]]) assert.equal(isAlreadyCaptured(response), false);
+});