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.d.ts

@@ -0,0 +1,24 @@
+import { StreamSource } from './_types.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
+ */
+declare function concatenateToResponse(sourcePromises: Promise<StreamSource>[], headersInit: HeadersInit): {
+    done: Promise<void>;
+    response: Response;
+};
+export { concatenateToResponse };