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/utils/createHeaders.d.ts

@@ -0,0 +1,17 @@
+import '../_version.js';
+/**
+ * This is a utility method that determines whether the current browser supports
+ * the features required to create streamed responses. Currently, it checks if
+ * [`ReadableStream`](https://developer.mozilla.org/en-US/docs/Web/API/ReadableStream/ReadableStream)
+ * is available.
+ *
+ * @private
+ * @param {HeadersInit} [headersInit] If there's no `Content-Type` specified,
+ * `'text/html'` will be used by default.
+ * @return {boolean} `true`, if the current browser meets the requirements for
+ * streaming responses, and `false` otherwise.
+ *
+ * @memberof workbox-streams
+ */
+declare function createHeaders(headersInit?: {}): Headers;
+export { createHeaders };