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/lodash.debounce/README.md

@@ -0,0 +1,18 @@
+# lodash.debounce v4.0.8
+
+The [lodash](https://lodash.com/) method `_.debounce` exported as a [Node.js](https://nodejs.org/) module.
+
+## Installation
+
+Using npm:
+```bash
+$ {sudo -H} npm i -g npm
+$ npm i --save lodash.debounce
+```
+
+In Node.js:
+```js
+var debounce = require('lodash.debounce');
+```
+
+See the [documentation](https://lodash.com/docs#debounce) or [package source](https://github.com/lodash/lodash/blob/4.0.8-npm-packages/lodash.debounce) for more details.