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/unique-string/readme.md

@@ -0,0 +1,32 @@
+# unique-string [![Build Status](https://travis-ci.org/sindresorhus/unique-string.svg?branch=master)](https://travis-ci.org/sindresorhus/unique-string)
+
+> Generate a unique random string
+
+
+## Install
+
+```
+$ npm install unique-string
+```
+
+
+## Usage
+
+```js
+const uniqueString = require('unique-string');
+
+uniqueString();
+//=> 'b4de2a49c8ffa3fbee04446f045483b2'
+```
+
+
+## API
+
+### uniqueString()
+
+Returns a 32 character unique string. Matches the length of MD5, which is [unique enough](https://stackoverflow.com/a/2444336/64949) for non-crypto purposes.
+
+
+## License
+
+MIT © [Sindre Sorhus](https://sindresorhus.com)