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/superPropGet.js

@@ -0,0 +1,9 @@
+var get = require("./get.js");
+var getPrototypeOf = require("./getPrototypeOf.js");
+function _superPropGet(t, o, e, r) {
+  var p = get(getPrototypeOf(1 & r ? t.prototype : t), o, e);
+  return 2 & r && "function" == typeof p ? function (t) {
+    return p.apply(e, t);
+  } : p;
+}
+module.exports = _superPropGet, module.exports.__esModule = true, module.exports["default"] = module.exports;