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/2025/LengthOfArrayLike.js

@@ -0,0 +1,18 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+var isObject = require('es-object-atoms/isObject');
+
+var Get = require('./Get');
+var ToLength = require('./ToLength');
+
+// https://262.ecma-international.org/11.0/#sec-lengthofarraylike
+
+module.exports = function LengthOfArrayLike(obj) {
+	if (!isObject(obj)) {
+		throw new $TypeError('Assertion failed: `obj` must be an Object');
+	}
+	return ToLength(Get(obj, 'length'));
+};
+
+// TODO: use this all over