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/defaults.js

@@ -0,0 +1,9 @@
+function _defaults(e, r) {
+  for (var t = Object.getOwnPropertyNames(r), o = 0; o < t.length; o++) {
+    var n = t[o],
+      a = Object.getOwnPropertyDescriptor(r, n);
+    a && a.configurable && void 0 === e[n] && Object.defineProperty(e, n, a);
+  }
+  return e;
+}
+export { _defaults as default };