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-precaching/utils/generateURLVariations.d.ts

@@ -0,0 +1,13 @@
+import { PrecacheRouteOptions } from '../_types.js';
+import '../_version.js';
+/**
+ * Generator function that yields possible variations on the original URL to
+ * check, one at a time.
+ *
+ * @param {string} url
+ * @param {Object} options
+ *
+ * @private
+ * @memberof workbox-precaching
+ */
+export declare function generateURLVariations(url: string, { ignoreURLParametersMatching, directoryIndex, cleanURLs, urlManipulation, }?: PrecacheRouteOptions): Generator<string, void, unknown>;