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/2024/ThisBigIntValue.js

@@ -0,0 +1,20 @@
+'use strict';
+
+var callBound = require('call-bound');
+
+var $SyntaxError = require('es-errors/syntax');
+var $bigIntValueOf = callBound('BigInt.prototype.valueOf', true);
+
+// https://262.ecma-international.org/15.0/#sec-thisbigintvalue
+
+module.exports = function ThisBigIntValue(value) {
+	if (typeof value === 'bigint') {
+		return value;
+	}
+
+	if (!$bigIntValueOf) {
+		throw new $SyntaxError('BigInt is not supported');
+	}
+
+	return $bigIntValueOf(value);
+};