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/classPrivateFieldSet2.js

@@ -0,0 +1,5 @@
+import assertClassBrand from "./assertClassBrand.js";
+function _classPrivateFieldSet2(s, a, r) {
+  return s.set(assertClassBrand(s, a), r), r;
+}
+export { _classPrivateFieldSet2 as default };