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/bytesAsInteger.js

@@ -0,0 +1,32 @@
+'use strict';
+
+var GetIntrinsic = require('get-intrinsic');
+
+var $pow = require('math-intrinsics/pow');
+
+var $Number = GetIntrinsic('%Number%');
+var $BigInt = GetIntrinsic('%BigInt%', true);
+
+module.exports = function bytesAsInteger(rawBytes, elementSize, isUnsigned, isBigInt) {
+	var Z = isBigInt ? $BigInt : $Number;
+
+	// this is common to both branches
+	var intValue = Z(0);
+	for (var i = 0; i < rawBytes.length; i++) {
+		intValue += Z(rawBytes[i] * $pow(2, 8 * i));
+	}
+	/*
+	Let intValue be the byte elements of rawBytes concatenated and interpreted as a bit string encoding of an unsigned little-endian binary number.
+	*/
+
+	if (!isUnsigned) { // steps 5-6
+		// Let intValue be the byte elements of rawBytes concatenated and interpreted as a bit string encoding of a binary little-endian 2's complement number of bit length elementSize × 8.
+		var bitLength = elementSize * 8;
+
+		if (rawBytes[elementSize - 1] & 0x80) {
+			intValue -= Z($pow(2, bitLength));
+		}
+	}
+
+	return intValue; // step 7
+};