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-strategies/utils/messages.js

@@ -0,0 +1,20 @@
+/*
+  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 { logger } from 'workbox-core/_private/logger.js';
+import { getFriendlyURL } from 'workbox-core/_private/getFriendlyURL.js';
+import '../_version.js';
+export const messages = {
+    strategyStart: (strategyName, request) => `Using ${strategyName} to respond to '${getFriendlyURL(request.url)}'`,
+    printFinalResponse: (response) => {
+        if (response) {
+            logger.groupCollapsed(`View the final response here.`);
+            logger.log(response || '[No response returned]');
+            logger.groupEnd();
+        }
+    },
+};