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/es-abstract-get/test/isPropertyKey.js

@@ -0,0 +1,26 @@
+'use strict';
+
+var test = require('tape');
+var forEach = require('for-each');
+var v = require('es-value-fixtures');
+var debug = require('object-inspect');
+
+var isPropertyKey = require('../isPropertyKey');
+
+/** @import { PropertyKey } from '../isPropertyKey' */
+
+test('isPropertyKey', function (t) {
+	forEach(v.nonPropertyKeys, function (nonPropertyKey) {
+		t.equal(isPropertyKey(nonPropertyKey), false, debug(nonPropertyKey) + ' is not a Property Key');
+	});
+
+	var propertyKeys = /** @type {PropertyKey[]} */ ([]).concat(
+		v.strings,
+		v.hasSymbols ? v.symbols : []
+	);
+	forEach(propertyKeys, function (propertyKey) {
+		t.equal(isPropertyKey(propertyKey), true, debug(propertyKey) + ' is a Property Key');
+	});
+
+	t.end();
+});