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/2020/StringToBigInt.js

@@ -0,0 +1,23 @@
+'use strict';
+
+var GetIntrinsic = require('get-intrinsic');
+
+var $BigInt = GetIntrinsic('%BigInt%', true);
+var $TypeError = require('es-errors/type');
+var $SyntaxError = require('es-errors/syntax');
+
+// https://262.ecma-international.org/11.0/#sec-stringtobigint
+
+module.exports = function StringToBigInt(argument) {
+	if (typeof argument !== 'string') {
+		throw new $TypeError('`argument` must be a string');
+	}
+	if (!$BigInt) {
+		throw new $SyntaxError('BigInts are not supported in this environment');
+	}
+	try {
+		return $BigInt(argument);
+	} catch (e) {
+		return NaN;
+	}
+};