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/2016/SameValueNonNumber.js

@@ -0,0 +1,14 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+
+var SameValue = require('./SameValue');
+
+// https://262.ecma-international.org/7.0/#sec-samevaluenonnumber
+
+module.exports = function SameValueNonNumber(x, y) {
+	if (typeof x === 'number' || typeof x !== typeof y) {
+		throw new $TypeError('SameValueNonNumber requires two non-number values of the same type.');
+	}
+	return SameValue(x, y);
+};