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/addPlugins.js

@@ -0,0 +1,21 @@
+/*
+  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 { getOrCreatePrecacheController } from './utils/getOrCreatePrecacheController.js';
+import './_version.js';
+/**
+ * Adds plugins to the precaching strategy.
+ *
+ * @param {Array<Object>} plugins
+ *
+ * @memberof workbox-precaching
+ */
+function addPlugins(plugins) {
+    const precacheController = getOrCreatePrecacheController();
+    precacheController.strategy.plugins.push(...plugins);
+}
+export { addPlugins };