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/2018/ToIndex.js

@@ -0,0 +1,24 @@
+'use strict';
+
+var $RangeError = require('es-errors/range');
+
+var ToInteger = require('./ToInteger');
+var ToLength = require('./ToLength');
+var SameValueZero = require('./SameValueZero');
+
+// https://262.ecma-international.org/8.0/#sec-toindex
+
+module.exports = function ToIndex(value) {
+	if (typeof value === 'undefined') {
+		return 0;
+	}
+	var integerIndex = ToInteger(value);
+	if (integerIndex < 0) {
+		throw new $RangeError('index must be >= 0');
+	}
+	var index = ToLength(integerIndex);
+	if (!SameValueZero(integerIndex, index)) {
+		throw new $RangeError('index must be >= 0 and < 2 ** 53 - 1');
+	}
+	return index;
+};