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/cjs/react.shared-subset.development.js

@@ -0,0 +1,20 @@
+/**
+ * @license React
+ * react.shared-subset.development.js
+ *
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+'use strict';
+
+if (process.env.NODE_ENV !== "production") {
+  (function() {
+'use strict';
+
+// eslint-disable-next-line react-internal/prod-error-codes
+throw new Error('This entry point is not yet supported outside of experimental channels');
+  })();
+}