Add sync error logging, extend error flash, fix unload saves

- Log workforce sync errors to backend stdout so they appear in docker logs
- Extend error flash from 5s to 15s so errors are readable
- Replace sendBeacon (POST-only) with keepalive fetch (PUT) on unload —
  sendBeacon was hitting 404s on all three unload saves

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/node_modules/workbox-streams/concatenateToResponse.js

@@ -0,0 +1,34 @@
+/*
+  Copyright 2018 Google LLC
+
+  Use of this source code is governed by an MIT-style
+  license that can be found in the LICENSE file or at
+  https://opensource.org/licenses/MIT.
+*/
+import { createHeaders } from './utils/createHeaders.js';
+import { concatenate } from './concatenate.js';
+import './_version.js';
+/**
+ * Takes multiple source Promises, each of which could resolve to a Response, a
+ * ReadableStream, or a [BodyInit](https://fetch.spec.whatwg.org/#bodyinit),
+ * along with a
+ * [HeadersInit](https://fetch.spec.whatwg.org/#typedefdef-headersinit).
+ *
+ * Returns an object exposing a Response whose body consists of each individual
+ * stream's data returned in sequence, along with a Promise which signals when
+ * the stream is finished (useful for passing to a FetchEvent's waitUntil()).
+ *
+ * @param {Array<Promise<workbox-streams.StreamSource>>} sourcePromises
+ * @param {HeadersInit} [headersInit] If there's no `Content-Type` specified,
+ * `'text/html'` will be used by default.
+ * @return {Object<{done: Promise, response: Response}>}
+ *
+ * @memberof workbox-streams
+ */
+function concatenateToResponse(sourcePromises, headersInit) {
+    const { done, stream } = concatenate(sourcePromises);
+    const headers = createHeaders(headersInit);
+    const response = new Response(stream, { headers });
+    return { done, response };
+}
+export { concatenateToResponse };