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/slicedToArray.js

@@ -0,0 +1,8 @@
+import arrayWithHoles from "./arrayWithHoles.js";
+import iterableToArrayLimit from "./iterableToArrayLimit.js";
+import unsupportedIterableToArray from "./unsupportedIterableToArray.js";
+import nonIterableRest from "./nonIterableRest.js";
+function _slicedToArray(r, e) {
+  return arrayWithHoles(r) || iterableToArrayLimit(r, e) || unsupportedIterableToArray(r, e) || nonIterableRest();
+}
+export { _slicedToArray as default };