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/README.md

@@ -0,0 +1,37 @@
+# `react`
+
+React is a JavaScript library for creating user interfaces.
+
+The `react` package contains only the functionality necessary to define React components. It is typically used together with a React renderer like `react-dom` for the web, or `react-native` for the native environments.
+
+**Note:** by default, React will be in development mode. The development version includes extra warnings about common mistakes, whereas the production version includes extra performance optimizations and strips all error messages. Don't forget to use the [production build](https://reactjs.org/docs/optimizing-performance.html#use-the-production-build) when deploying your application.
+
+## Usage
+
+```js
+import { useState } from 'react';
+import { createRoot } from 'react-dom/client';
+
+function Counter() {
+  const [count, setCount] = useState(0);
+  return (
+    <>
+      <h1>{count}</h1>
+      <button onClick={() => setCount(count + 1)}>
+        Increment
+      </button>
+    </>
+  );
+}
+
+const root = createRoot(document.getElementById('root'));
+root.render(<App />);
+```
+
+## Documentation
+
+See https://reactjs.org/
+
+## API
+
+See https://reactjs.org/docs/react-api.html