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/precache.d.ts

@@ -0,0 +1,23 @@
+import { PrecacheEntry } from './_types.js';
+import './_version.js';
+/**
+ * Adds items to the precache list, removing any duplicates and
+ * stores the files in the
+ * {@link workbox-core.cacheNames|"precache cache"} when the service
+ * worker installs.
+ *
+ * This method can be called multiple times.
+ *
+ * Please note: This method **will not** serve any of the cached files for you.
+ * It only precaches files. To respond to a network request you call
+ * {@link workbox-precaching.addRoute}.
+ *
+ * If you have a single array of files to precache, you can just call
+ * {@link workbox-precaching.precacheAndRoute}.
+ *
+ * @param {Array<Object|string>} [entries=[]] Array of entries to precache.
+ *
+ * @memberof workbox-precaching
+ */
+declare function precache(entries: Array<PrecacheEntry | string>): void;
+export { precache };