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/arrayLikeToArray.js

@@ -0,0 +1,6 @@
+function _arrayLikeToArray(r, a) {
+  (null == a || a > r.length) && (a = r.length);
+  for (var e = 0, n = Array(a); e < a; e++) n[e] = r[e];
+  return n;
+}
+export { _arrayLikeToArray as default };