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/helpers/integerToNBytes.js

@@ -0,0 +1,28 @@
+'use strict';
+
+var GetIntrinsic = require('get-intrinsic');
+
+var $Number = GetIntrinsic('%Number%');
+var $BigInt = GetIntrinsic('%BigInt%', true);
+
+module.exports = function integerToNBytes(intValue, n, isLittleEndian) {
+	var Z = typeof intValue === 'bigint' ? $BigInt : $Number;
+	/*
+	if (intValue >= 0) { // step 3.d
+		// Let rawBytes be a List containing the n-byte binary encoding of intValue. If isLittleEndian is false, the bytes are ordered in big endian order. Otherwise, the bytes are ordered in little endian order.
+	} else { // step 3.e
+		// Let rawBytes be a List containing the n-byte binary 2's complement encoding of intValue. If isLittleEndian is false, the bytes are ordered in big endian order. Otherwise, the bytes are ordered in little endian order.
+	}
+    */
+	if (intValue < 0) {
+		intValue >>>= 0; // eslint-disable-line no-param-reassign
+	}
+
+	var rawBytes = [];
+	for (var i = 0; i < n; i++) {
+		rawBytes[isLittleEndian ? i : n - 1 - i] = $Number(intValue & Z(0xFF));
+		intValue >>= Z(8); // eslint-disable-line no-param-reassign
+	}
+
+	return rawBytes; // step 4
+};