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/concatenate.d.ts

@@ -0,0 +1,20 @@
+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).
+ *
+ * Returns an object exposing a ReadableStream with 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
+ * @return {Object<{done: Promise, stream: ReadableStream}>}
+ *
+ * @memberof workbox-streams
+ */
+declare function concatenate(sourcePromises: Promise<StreamSource>[]): {
+    done: Promise<void>;
+    stream: ReadableStream;
+};
+export { concatenate };