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-navigation-preload/enable.d.ts

@@ -0,0 +1,13 @@
+import './_version.js';
+/**
+ * If the browser supports Navigation Preload, then this will enable it.
+ *
+ * @param {string} [headerValue] Optionally, allows developers to
+ * [override](https://developers.google.com/web/updates/2017/02/navigation-preload#changing_the_header)
+ * the value of the `Service-Worker-Navigation-Preload` header which will be
+ * sent to the server when making the navigation request.
+ *
+ * @memberof workbox-navigation-preload
+ */
+declare function enable(headerValue?: string): void;
+export { enable };