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/utils/WorkboxEvent.js

@@ -0,0 +1,20 @@
+/*
+  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 '../_version.js';
+/**
+ * A minimal `Event` subclass shim.
+ * This doesn't *actually* subclass `Event` because not all browsers support
+ * constructable `EventTarget`, and using a real `Event` will error.
+ * @private
+ */
+export class WorkboxEvent {
+    constructor(type, props) {
+        this.type = type;
+        Object.assign(this, props);
+    }
+}