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-plugin-polyfill-regenerator/README.md

@@ -0,0 +1,28 @@
+# babel-plugin-polyfill-regenerator
+
+## Install
+
+Using npm:
+
+```sh
+npm install --save-dev babel-plugin-polyfill-regenerator
+```
+
+or using yarn:
+
+```sh
+yarn add babel-plugin-polyfill-regenerator --dev
+```
+
+## Usage
+
+Add this plugin to your Babel configuration:
+
+```json
+{
+  "plugins": [["polyfill-regenerator", { "method": "usage-global" }]]
+}
+```
+
+This package supports the `usage-pure`, `usage-global`, and `entry-global` methods.
+When `entry-global` is used, it replaces imports to `regenerator-runtime`.