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/has-property-descriptors/index.js

@@ -0,0 +1,22 @@
+'use strict';
+
+var $defineProperty = require('es-define-property');
+
+var hasPropertyDescriptors = function hasPropertyDescriptors() {
+	return !!$defineProperty;
+};
+
+hasPropertyDescriptors.hasArrayLengthDefineBug = function hasArrayLengthDefineBug() {
+	// node v0.6 has a bug where array lengths can be Set but not Defined
+	if (!$defineProperty) {
+		return null;
+	}
+	try {
+		return $defineProperty([], 'length', { value: 1 }).length !== 1;
+	} catch (e) {
+		// In Firefox 4-22, defining length on an array throws an exception.
+		return true;
+	}
+};
+
+module.exports = hasPropertyDescriptors;