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/5/ToNumber.js

@@ -0,0 +1,34 @@
+'use strict';
+
+var ToPrimitive = require('./ToPrimitive');
+
+var callBound = require('call-bound');
+
+var $replace = callBound('String.prototype.replace');
+
+var safeRegexTester = require('safe-regex-test');
+
+var isNonDecimal = safeRegexTester(/^0[ob]|^[+-]0x/);
+
+var $Number = Number;
+
+// http://262.ecma-international.org/5.1/#sec-9.3
+
+module.exports = function ToNumber(value) {
+	var prim = ToPrimitive(value, $Number);
+	if (typeof prim !== 'string') {
+		return $Number(prim);
+	}
+
+	var trimmed = $replace(
+		prim,
+		// eslint-disable-next-line no-control-regex
+		/^[ \t\x0b\f\xa0\ufeff\n\r\u2028\u2029\u1680\u180e\u2000\u2001\u2002\u2003\u2004\u2005\u2006\u2007\u2008\u2009\u200a\u202f\u205f\u3000\u0085]+|[ \t\x0b\f\xa0\ufeff\n\r\u2028\u2029\u1680\u180e\u2000\u2001\u2002\u2003\u2004\u2005\u2006\u2007\u2008\u2009\u200a\u202f\u205f\u3000\u0085]+$/g,
+		''
+	);
+	if (isNonDecimal(trimmed)) {
+		return NaN;
+	}
+
+	return +trimmed;
+};