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/esm/construct.js

@@ -0,0 +1,10 @@
+import isNativeReflectConstruct from "./isNativeReflectConstruct.js";
+import setPrototypeOf from "./setPrototypeOf.js";
+function _construct(t, e, r) {
+  if (isNativeReflectConstruct()) return Reflect.construct.apply(null, arguments);
+  var o = [null];
+  o.push.apply(o, e);
+  var p = new (t.bind.apply(t, o))();
+  return r && setPrototypeOf(p, r.prototype), p;
+}
+export { _construct as default };