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/IfAbruptCloseIterator.js

@@ -0,0 +1,19 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+
+var CompletionRecord = require('./CompletionRecord');
+var IteratorClose = require('./IteratorClose');
+
+// https://262.ecma-international.org/16.0/#sec-ifabruptcloseiterator
+
+module.exports = function IfAbruptCloseIterator(value, iteratorRecord) {
+	if (!(value instanceof CompletionRecord)) {
+		throw new $TypeError('Assertion failed: `value` must be a Completion Record'); // step 1
+	}
+	if (value.type() !== 'normal') {
+		return IteratorClose(iteratorRecord, value); // step 2
+	}
+
+	return value['!'](); // step 3
+};