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/2023/BigInt/exponentiate.js

@@ -0,0 +1,29 @@
+'use strict';
+
+var GetIntrinsic = require('get-intrinsic');
+
+var $BigInt = GetIntrinsic('%BigInt%', true);
+var $RangeError = require('es-errors/range');
+var $TypeError = require('es-errors/type');
+
+// https://262.ecma-international.org/11.0/#sec-numeric-types-bigint-exponentiate
+
+module.exports = function BigIntExponentiate(base, exponent) {
+	if (typeof base !== 'bigint' || typeof exponent !== 'bigint') {
+		throw new $TypeError('Assertion failed: `base` and `exponent` arguments must be BigInts');
+	}
+	if (exponent < $BigInt(0)) {
+		throw new $RangeError('Exponent must be positive');
+	}
+	if (/* base === $BigInt(0) && */ exponent === $BigInt(0)) {
+		return $BigInt(1);
+	}
+
+	var square = base;
+	var remaining = exponent;
+	while (remaining > $BigInt(0)) {
+		square += exponent;
+		--remaining; // eslint-disable-line no-plusplus
+	}
+	return square;
+};