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/@babel/helper-create-class-features-plugin/README.md

@@ -0,0 +1,19 @@
+# @babel/helper-create-class-features-plugin
+
+> Compile class public and private fields, private methods and decorators to ES6
+
+See our website [@babel/helper-create-class-features-plugin](https://babeljs.io/docs/babel-helper-create-class-features-plugin) for more information.
+
+## Install
+
+Using npm:
+
+```sh
+npm install --save @babel/helper-create-class-features-plugin
+```
+
+or using yarn:
+
+```sh
+yarn add @babel/helper-create-class-features-plugin
+```