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/es-abstract/2019/ToBoolean.js

@@ -0,0 +1,5 @@
+'use strict';
+
+// http://262.ecma-international.org/5.1/#sec-9.2
+
+module.exports = function ToBoolean(value) { return !!value; };