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-broadcast-update/responsesAreSame.d.ts

@@ -0,0 +1,14 @@
+import './_version.js';
+/**
+ * Given two `Response's`, compares several header values to see if they are
+ * the same or not.
+ *
+ * @param {Response} firstResponse
+ * @param {Response} secondResponse
+ * @param {Array<string>} headersToCheck
+ * @return {boolean}
+ *
+ * @memberof workbox-broadcast-update
+ */
+declare const responsesAreSame: (firstResponse: Response, secondResponse: Response, headersToCheck: string[]) => boolean;
+export { responsesAreSame };