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/2023/IteratorNext.js

@@ -0,0 +1,28 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+var isObject = require('es-object-atoms/isObject');
+
+var Call = require('./Call');
+
+var isIteratorRecord = require('../helpers/records/iterator-record-2023');
+
+// https://262.ecma-international.org/14.0/#sec-iteratornext
+
+module.exports = function IteratorNext(iteratorRecord) {
+	if (!isIteratorRecord(iteratorRecord)) {
+		throw new $TypeError('Assertion failed: `iteratorRecord` must be an Iterator Record'); // step 1
+	}
+
+	var result;
+	if (arguments.length < 2) { // step 1
+		result = Call(iteratorRecord['[[NextMethod]]'], iteratorRecord['[[Iterator]]']); // step 1.a
+	} else { // step 2
+		result = Call(iteratorRecord['[[NextMethod]]'], iteratorRecord['[[Iterator]]'], [arguments[1]]); // step 2.a
+	}
+
+	if (!isObject(result)) {
+		throw new $TypeError('iterator next must return an object'); // step 3
+	}
+	return result; // step 4
+};