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/regeneratorKeys.js

@@ -0,0 +1,10 @@
+function _regeneratorKeys(e) {
+  var n = Object(e),
+    r = [];
+  for (var t in n) r.unshift(t);
+  return function e() {
+    for (; r.length;) if ((t = r.pop()) in n) return e.value = t, e.done = !1, e;
+    return e.done = !0, e;
+  };
+}
+module.exports = _regeneratorKeys, module.exports.__esModule = true, module.exports["default"] = module.exports;