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/2022/BigInt/toString.js

@@ -0,0 +1,16 @@
+'use strict';
+
+var GetIntrinsic = require('get-intrinsic');
+
+var $String = GetIntrinsic('%String%');
+var $TypeError = require('es-errors/type');
+
+// https://262.ecma-international.org/11.0/#sec-numeric-types-bigint-tostring
+
+module.exports = function BigIntToString(x) {
+	if (typeof x !== 'bigint') {
+		throw new $TypeError('Assertion failed: `x` must be a BigInt');
+	}
+
+	return $String(x);
+};