Checkout: forward only the checkout fields, show clear errors, fix the contact form (#16)

Co-authored-by: Michael Czechowski <mail@librete.ch>

utils/captureResponse.ts

@@ -0,0 +1,12 @@
+// The answer of POST /api/orders/:uuid/capture/:paypalOrderId: the paid order, or, when the order is already paid with this
+// PayPal order (a repeated capture), { success: true, alreadyCaptured: true } (libreshop/cms src/api/order/controllers/order.ts).
+// Pure: no Nuxt or Vue imports, tested in tests/unit/captureResponse.test.ts.
+
+/** The CMS's answer to a repeated capture: the payment went through before, and the order is paid. */
+export type AlreadyCaptured = { success: true; alreadyCaptured: true };
+
+export const isAlreadyCaptured = (response: unknown): response is AlreadyCaptured =>
+	typeof response === "object" &&
+	response !== null &&
+	(response as Record<string, unknown>).success === true &&
+	(response as Record<string, unknown>).alreadyCaptured === true;