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/IteratorStep.js

@@ -0,0 +1,21 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+
+var IteratorComplete = require('./IteratorComplete');
+var IteratorNext = require('./IteratorNext');
+
+var isIteratorRecord = require('../helpers/records/iterator-record-2023');
+
+// https://262.ecma-international.org/14.0/#sec-iteratorstep
+
+module.exports = function IteratorStep(iteratorRecord) {
+	if (!isIteratorRecord(iteratorRecord)) {
+		throw new $TypeError('Assertion failed: `iteratorRecord` must be an Iterator Record'); // step 1
+	}
+
+	var result = IteratorNext(iteratorRecord); // step 1
+	var done = IteratorComplete(result); // step 2
+	return done === true ? false : result; // steps 3-4
+};
+