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/GetV.js

@@ -0,0 +1,23 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+
+var inspect = require('object-inspect');
+
+var isPropertyKey = require('../helpers/isPropertyKey');
+// var ToObject = require('./ToObject');
+
+// https://262.ecma-international.org/6.0/#sec-getv
+
+module.exports = function GetV(V, P) {
+	// 7.3.2.1
+	if (!isPropertyKey(P)) {
+		throw new $TypeError('Assertion failed: P is not a Property Key, got ' + inspect(P));
+	}
+
+	// 7.3.2.2-3
+	// var O = ToObject(V);
+
+	// 7.3.2.4
+	return V[P];
+};