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/extends.js

@@ -0,0 +1,10 @@
+function _extends() {
+  return _extends = Object.assign ? Object.assign.bind() : function (n) {
+    for (var e = 1; e < arguments.length; e++) {
+      var t = arguments[e];
+      for (var r in t) ({}).hasOwnProperty.call(t, r) && (n[r] = t[r]);
+    }
+    return n;
+  }, _extends.apply(null, arguments);
+}
+export { _extends as default };