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/IsExtensible.js

@@ -0,0 +1,18 @@
+'use strict';
+
+var GetIntrinsic = require('get-intrinsic');
+
+var $preventExtensions = GetIntrinsic('%Object.preventExtensions%', true);
+var $isExtensible = GetIntrinsic('%Object.isExtensible%', true);
+
+var isPrimitive = require('../helpers/isPrimitive');
+
+// https://262.ecma-international.org/6.0/#sec-isextensible-o
+
+module.exports = $preventExtensions
+	? function IsExtensible(obj) {
+		return !isPrimitive(obj) && $isExtensible(obj);
+	}
+	: function IsExtensible(obj) {
+		return !isPrimitive(obj);
+	};