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/plugins/cacheOkAndOpaquePlugin.js

@@ -0,0 +1,26 @@
+/*
+  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 '../_version.js';
+export const cacheOkAndOpaquePlugin = {
+    /**
+     * Returns a valid response (to allow caching) if the status is 200 (OK) or
+     * 0 (opaque).
+     *
+     * @param {Object} options
+     * @param {Response} options.response
+     * @return {Response|null}
+     *
+     * @private
+     */
+    cacheWillUpdate: async ({ response }) => {
+        if (response.status === 200 || response.status === 0) {
+            return response;
+        }
+        return null;
+    },
+};