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/superPropSet.js

@@ -0,0 +1,6 @@
+var set = require("./set.js");
+var getPrototypeOf = require("./getPrototypeOf.js");
+function _superPropSet(t, e, o, r, p, f) {
+  return set(getPrototypeOf(f ? t.prototype : t), e, o, r, p);
+}
+module.exports = _superPropSet, module.exports.__esModule = true, module.exports["default"] = module.exports;