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/objectWithoutPropertiesLoose.js

@@ -0,0 +1,10 @@
+function _objectWithoutPropertiesLoose(r, e) {
+  if (null == r) return {};
+  var t = {};
+  for (var n in r) if ({}.hasOwnProperty.call(r, n)) {
+    if (-1 !== e.indexOf(n)) continue;
+    t[n] = r[n];
+  }
+  return t;
+}
+module.exports = _objectWithoutPropertiesLoose, module.exports.__esModule = true, module.exports["default"] = module.exports;