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/2019/Invoke.js

@@ -0,0 +1,22 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+
+var Call = require('./Call');
+var IsArray = require('./IsArray');
+var GetV = require('./GetV');
+var isPropertyKey = require('../helpers/isPropertyKey');
+
+// https://262.ecma-international.org/6.0/#sec-invoke
+
+module.exports = function Invoke(O, P) {
+	if (!isPropertyKey(P)) {
+		throw new $TypeError('Assertion failed: P must be a Property Key');
+	}
+	var argumentsList = arguments.length > 2 ? arguments[2] : [];
+	if (!IsArray(argumentsList)) {
+		throw new $TypeError('Assertion failed: optional `argumentsList`, if provided, must be a List');
+	}
+	var func = GetV(O, P);
+	return Call(func, O, argumentsList);
+};