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-routing/utils/getOrCreateDefaultRouter.d.ts

@@ -0,0 +1,10 @@
+import { Router } from '../Router.js';
+import '../_version.js';
+/**
+ * Creates a new, singleton Router instance if one does not exist. If one
+ * does already exist, that instance is returned.
+ *
+ * @private
+ * @return {Router}
+ */
+export declare const getOrCreateDefaultRouter: () => Router;