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/runtime/helpers/nullishReceiverError.js

@@ -0,0 +1,4 @@
+function _nullishReceiverError(r) {
+  throw new TypeError("Cannot set property of null or undefined.");
+}
+module.exports = _nullishReceiverError, module.exports.__esModule = true, module.exports["default"] = module.exports;