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/deleteOutdatedCaches.d.ts

@@ -0,0 +1,21 @@
+import '../_version.js';
+/**
+ * Cleans up incompatible precaches that were created by older versions of
+ * Workbox, by a service worker registered under the current scope.
+ *
+ * This is meant to be called as part of the `activate` event.
+ *
+ * This should be safe to use as long as you don't include `substringToFind`
+ * (defaulting to `-precache-`) in your non-precache cache names.
+ *
+ * @param {string} currentPrecacheName The cache name currently in use for
+ * precaching. This cache won't be deleted.
+ * @param {string} [substringToFind='-precache-'] Cache names which include this
+ * substring will be deleted (excluding `currentPrecacheName`).
+ * @return {Array<string>} A list of all the cache names that were deleted.
+ *
+ * @private
+ * @memberof workbox-precaching
+ */
+declare const deleteOutdatedCaches: (currentPrecacheName: string, substringToFind?: string) => Promise<string[]>;
+export { deleteOutdatedCaches };