fix(api): wrap invoice API responses with cors() helper

The order-action / order-block UI extensions are hosted on
extensions.shopifycdn.com and call our app via fetch(). Without CORS
headers the browser blocked the response. authenticate.admin already
returns a cors helper and handles OPTIONS preflight - wrap every
Response with it.

app/routes/api.orders.$orderId.invoice.tsx

@@ -12,7 +12,7 @@ import { cancelAndReissue } from "../services/invoice/cancelAndReissue.server";
  * normalises it.
  */
 export const loader = async ({ request, params }: LoaderFunctionArgs) => {
-  const { session } = await authenticate.admin(request);
+  const { session, cors } = await authenticate.admin(request);
   const orderId = requireOrderId(params);
   const orderGid = orderId.startsWith("gid://")
     ? orderId
@@ -24,16 +24,18 @@ export const loader = async ({ request, params }: LoaderFunctionArgs) => {
   });
   const latest = invoices.find((i) => i.kind === "invoice" && !i.cancelledAt);
 
-  return {
-    latest: latest ? serialise(latest) : null,
-    history: invoices.map(serialise),
-  };
+  return cors(
+    Response.json({
+      latest: latest ? serialise(latest) : null,
+      history: invoices.map(serialise),
+    }),
+  );
 };
 
 export const action = async ({ request, params }: ActionFunctionArgs) => {
-  const { admin, session } = await authenticate.admin(request);
+  const { admin, session, cors } = await authenticate.admin(request);
   if (request.method !== "POST") {
-    return new Response("Method Not Allowed", { status: 405 });
+    return cors(new Response("Method Not Allowed", { status: 405 }));
   }
   const orderId = requireOrderId(params);
   const url = new URL(request.url);
@@ -55,7 +57,7 @@ export const action = async ({ request, params }: ActionFunctionArgs) => {
         admin,
         orderId,
       });
-      return { ok: true, op, ...result };
+      return cors(Response.json({ ok: true, op, ...result }));
     }
 
     const result = await generateInvoice({
@@ -63,11 +65,11 @@ export const action = async ({ request, params }: ActionFunctionArgs) => {
       admin,
       orderId,
     });
-    return { ok: true, op: "generate", ...result };
+    return cors(Response.json({ ok: true, op: "generate", ...result }));
   } catch (err) {
     const message = err instanceof Error ? err.message : String(err);
     console.error("invoice action failed:", err);
-    return Response.json({ ok: false, error: message }, { status: 400 });
+    return cors(Response.json({ ok: false, error: message }, { status: 400 }));
   }
 };