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-window/messageSW.d.ts

@@ -0,0 +1,17 @@
+import './_version.js';
+/**
+ * Sends a data object to a service worker via `postMessage` and resolves with
+ * a response (if any).
+ *
+ * A response can be set in a message handler in the service worker by
+ * calling `event.ports[0].postMessage(...)`, which will resolve the promise
+ * returned by `messageSW()`. If no response is set, the promise will not
+ * resolve.
+ *
+ * @param {ServiceWorker} sw The service worker to send the message to.
+ * @param {Object} data An object to send to the service worker.
+ * @return {Promise<Object|undefined>}
+ * @memberof workbox-window
+ */
+declare function messageSW(sw: ServiceWorker, data: {}): Promise<any>;
+export { messageSW };