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-core/index.js

@@ -0,0 +1,24 @@
+/*
+  Copyright 2018 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 { registerQuotaErrorCallback } from './registerQuotaErrorCallback.js';
+import * as _private from './_private.js';
+import { cacheNames } from './cacheNames.js';
+import { copyResponse } from './copyResponse.js';
+import { clientsClaim } from './clientsClaim.js';
+import { setCacheNameDetails } from './setCacheNameDetails.js';
+import { skipWaiting } from './skipWaiting.js';
+import './_version.js';
+/**
+ * All of the Workbox service worker libraries use workbox-core for shared
+ * code as well as setting default values that need to be shared (like cache
+ * names).
+ *
+ * @module workbox-core
+ */
+export { _private, cacheNames, clientsClaim, copyResponse, registerQuotaErrorCallback, setCacheNameDetails, skipWaiting, };
+export * from './types.js';