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-background-sync/StorableRequest.js

@@ -0,0 +1,12 @@
+/*
+  Copyright 2021 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 './_version.js';
+// This is a temporary workaround to expose something from ./lib/ via our
+// top-level public API.
+// TODO: In Workbox v7, move the actual code from ./lib/ to this file.
+export { StorableRequest } from './lib/StorableRequest';