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/get-own-enumerable-property-symbols/lib/index.d.ts

@@ -0,0 +1,2 @@
+declare const _default: (object: object) => symbol[];
+export default _default;

frontend/node_modules/get-own-enumerable-property-symbols/lib/index.js

@@ -0,0 +1,6 @@
+"use strict";
+Object.defineProperty(exports, "__esModule", { value: true });
+exports.default = (object) => Object
+    .getOwnPropertySymbols(object)
+    .filter((keySymbol) => Object.prototype.propertyIsEnumerable.call(object, keySymbol));
+//# sourceMappingURL=index.js.map

frontend/node_modules/get-own-enumerable-property-symbols/lib/index.js.map

@@ -0,0 +1 @@
+{"version":3,"file":"index.js","sourceRoot":"","sources":["../src/index.ts"],"names":[],"mappings":";;AAAA,kBAAe,CAAC,MAAc,EAAY,EAAE,CAAC,MAAM;KAChD,qBAAqB,CAAC,MAAM,CAAC;KAC7B,MAAM,CAAC,CAAC,SAAS,EAAW,EAAE,CAAC,MAAM,CAAC,SAAS,CAAC,oBAAoB,CAAC,IAAI,CAAC,MAAM,EAAE,SAAS,CAAC,CAAC,CAAA"}