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/react-router-dom/dist/main.js

@@ -0,0 +1,19 @@
+/**
+ * React Router DOM v6.30.4
+ *
+ * Copyright (c) Remix Software Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE.md file in the root directory of this source tree.
+ *
+ * @license MIT
+ */
+'use strict';
+
+/* eslint-env node */
+
+if (process.env.NODE_ENV === "production") {
+  module.exports = require("./umd/react-router-dom.production.min.js");
+} else {
+  module.exports = require("./umd/react-router-dom.development.js");
+}