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/2022/InstallErrorCause.js

@@ -0,0 +1,21 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+var isObject = require('es-object-atoms/isObject');
+
+var CreateNonEnumerableDataPropertyOrThrow = require('./CreateNonEnumerableDataPropertyOrThrow');
+var Get = require('./Get');
+var HasProperty = require('./HasProperty');
+
+// https://262.ecma-international.org/13.0/#sec-installerrorcause
+
+module.exports = function InstallErrorCause(O, options) {
+	if (!isObject(O)) {
+		throw new $TypeError('Assertion failed: Type(O) is not Object');
+	}
+
+	if (isObject(options) && HasProperty(options, 'cause')) {
+		var cause = Get(options, 'cause');
+		CreateNonEnumerableDataPropertyOrThrow(O, 'cause', cause);
+	}
+};