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/2015/IsRegExp.js

@@ -0,0 +1,25 @@
+'use strict';
+
+var GetIntrinsic = require('get-intrinsic');
+
+var $match = GetIntrinsic('%Symbol.match%', true);
+
+var hasRegExpMatcher = require('is-regex');
+var isObject = require('es-object-atoms/isObject');
+
+var ToBoolean = require('./ToBoolean');
+
+// https://262.ecma-international.org/6.0/#sec-isregexp
+
+module.exports = function IsRegExp(argument) {
+	if (!isObject(argument)) {
+		return false;
+	}
+	if ($match) {
+		var isRegExp = argument[$match];
+		if (typeof isRegExp !== 'undefined') {
+			return ToBoolean(isRegExp);
+		}
+	}
+	return hasRegExpMatcher(argument);
+};