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/getCacheKeyForURL.d.ts

@@ -0,0 +1,22 @@
+import './_version.js';
+/**
+ * Takes in a URL, and returns the corresponding URL that could be used to
+ * lookup the entry in the precache.
+ *
+ * If a relative URL is provided, the location of the service worker file will
+ * be used as the base.
+ *
+ * For precached entries without revision information, the cache key will be the
+ * same as the original URL.
+ *
+ * For precached entries with revision information, the cache key will be the
+ * original URL with the addition of a query parameter used for keeping track of
+ * the revision info.
+ *
+ * @param {string} url The URL whose cache key to look up.
+ * @return {string} The cache key that corresponds to that URL.
+ *
+ * @memberof workbox-precaching
+ */
+declare function getCacheKeyForURL(url: string): string | undefined;
+export { getCacheKeyForURL };