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/2023/CreateHTML.js

@@ -0,0 +1,27 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+
+var callBound = require('call-bound');
+
+var $replace = callBound('String.prototype.replace');
+
+var RequireObjectCoercible = require('./RequireObjectCoercible');
+var ToString = require('./ToString');
+
+// https://262.ecma-international.org/6.0/#sec-createhtml
+
+module.exports = function CreateHTML(string, tag, attribute, value) {
+	if (typeof tag !== 'string' || typeof attribute !== 'string') {
+		throw new $TypeError('Assertion failed: `tag` and `attribute` must be strings');
+	}
+	var str = RequireObjectCoercible(string);
+	var S = ToString(str);
+	var p1 = '<' + tag;
+	if (attribute !== '') {
+		var V = ToString(value);
+		var escapedV = $replace(V, /\x22/g, '&quot;');
+		p1 += '\x20' + attribute + '\x3D\x22' + escapedV + '\x22';
+	}
+	return p1 + '>' + S + '</' + tag + '>';
+};