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/README.md

@@ -0,0 +1,16 @@
+# React Router
+
+The `react-router` package is the heart of [React Router](https://github.com/remix-run/react-router) and provides all
+the core functionality for both
+[`react-router-dom`](https://github.com/remix-run/react-router/tree/main/packages/react-router-dom)
+and
+[`react-router-native`](https://github.com/remix-run/react-router/tree/main/packages/react-router-native).
+
+If you're using React Router, you should never `import` anything directly from
+the `react-router` package, but you should have everything you need in either
+`react-router-dom` or `react-router-native`. Both of those packages re-export
+everything from `react-router`.
+
+If you'd like to extend React Router and you know what you're doing, you should
+add `react-router` **as a peer dependency, not a regular dependency** in your
+package.