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/2024/IsTimeZoneOffsetString.js

@@ -0,0 +1,20 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+
+var regexTester = require('safe-regex-test');
+
+// https://tc39.es/ecma262/#sec-istimezoneoffsetstring
+
+// implementation taken from https://github.com/tc39/proposal-temporal/blob/21ee5b13f0672990c807475ba094092d19dd6dc5/polyfill/lib/ecmascript.mjs#L2140
+
+var OFFSET = /^([+\u2212-])([01][0-9]|2[0-3])(?::?([0-5][0-9])(?::?([0-5][0-9])(?:[.,](\d{1,9}))?)?)?$/;
+
+var testOffset = regexTester(OFFSET);
+
+module.exports = function IsTimeZoneOffsetString(offsetString) {
+	if (typeof offsetString !== 'string') {
+		throw new $TypeError('Assertion failed: `offsetString` must be a String');
+	}
+	return testOffset(offsetString);
+};