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-build/build/lib/escape-regexp.js

@@ -0,0 +1,15 @@
+"use strict";
+/*
+  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.
+*/
+Object.defineProperty(exports, "__esModule", { value: true });
+exports.escapeRegExp = void 0;
+// From https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions
+function escapeRegExp(str) {
+    return str.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
+}
+exports.escapeRegExp = escapeRegExp;