fix(ancpi): use form-data multipart for saveProductMetadataForBasketItem

Angular uses doPostAsFormMultipart — the save endpoint requires
multipart/form-data, not application/x-www-form-urlencoded.
Install form-data package and restore multipart upload.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/modules/parcel-sync/services/epay-client.ts

@@ -19,6 +19,7 @@ import axios, { type AxiosInstance } from "axios";
 import crypto from "crypto";
 import { wrapper } from "axios-cookiejar-support";
 import { CookieJar } from "tough-cookie";
+import FormData from "form-data";
 import type {
   EpayCartResponse,
   EpaySearchResult,
@@ -399,18 +400,17 @@ export class EpayClient {
       configurationUrl: "http://www.google.com",
     };
 
-    // Use URLSearchParams instead of multipart FormData
-    // (axios + cookie-jar has issues with native FormData)
-    const form = new URLSearchParams();
-    form.set("reqType", "saveProductMetadataForBasketItem");
-    form.set("productMetadataJSON", JSON.stringify(metadata));
+    // Must use multipart/form-data (Angular uses doPostAsFormMultipart)
+    const form = new FormData();
+    form.append("reqType", "saveProductMetadataForBasketItem");
+    form.append("productMetadataJSON", JSON.stringify(metadata));
 
     const response = await this.client.post(
       `${BASE_URL}/EpayJsonInterceptor.action`,
-      form.toString(),
+      form,
       {
         headers: {
-          "Content-Type": "application/x-www-form-urlencoded; charset=UTF-8",
+          ...form.getHeaders(),
           "X-Requested-With": "XMLHttpRequest",
         },
         timeout: DEFAULT_TIMEOUT_MS,