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/hasown/index.js

@@ -0,0 +1,8 @@
+'use strict';
+
+var call = Function.prototype.call;
+var $hasOwn = Object.prototype.hasOwnProperty;
+var bind = require('function-bind');
+
+/** @type {import('.')} */
+module.exports = bind.call(call, $hasOwn);