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/2016/IsGenericDescriptor.js

@@ -0,0 +1,26 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+
+var IsAccessorDescriptor = require('./IsAccessorDescriptor');
+var IsDataDescriptor = require('./IsDataDescriptor');
+
+var isPropertyDescriptor = require('../helpers/records/property-descriptor');
+
+// https://262.ecma-international.org/6.0/#sec-isgenericdescriptor
+
+module.exports = function IsGenericDescriptor(Desc) {
+	if (typeof Desc === 'undefined') {
+		return false;
+	}
+
+	if (!isPropertyDescriptor(Desc)) {
+		throw new $TypeError('Assertion failed: `Desc` must be a Property Descriptor');
+	}
+
+	if (!IsAccessorDescriptor(Desc) && !IsDataDescriptor(Desc)) {
+		return true;
+	}
+
+	return false;
+};