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-window/index.js

@@ -0,0 +1,16 @@
+/*
+  Copyright 2019 Google LLC
+
+  Use of this source code is governed by an MIT-style
+  license that can be found in the LICENSE file or at
+  https://opensource.org/licenses/MIT.
+*/
+import { messageSW } from './messageSW.js';
+import { Workbox } from './Workbox.js';
+import './_version.js';
+/**
+ * @module workbox-window
+ */
+export { messageSW, Workbox };
+// See https://github.com/GoogleChrome/workbox/issues/2770
+export * from './utils/WorkboxEvent.js';