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/toSetter.js

@@ -0,0 +1,10 @@
+function _toSetter(t, e, n) {
+  e || (e = []);
+  var r = e.length++;
+  return Object.defineProperty({}, "_", {
+    set: function set(o) {
+      e[r] = o, t.apply(n, e);
+    }
+  });
+}
+export { _toSetter as default };