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/CanonicalNumericIndexString.js

@@ -0,0 +1,19 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+
+var SameValue = require('./SameValue');
+var ToNumber = require('./ToNumber');
+var ToString = require('./ToString');
+
+// https://262.ecma-international.org/6.0/#sec-canonicalnumericindexstring
+
+module.exports = function CanonicalNumericIndexString(argument) {
+	if (typeof argument !== 'string') {
+		throw new $TypeError('Assertion failed: `argument` must be a String');
+	}
+	if (argument === '-0') { return -0; }
+	var n = ToNumber(argument);
+	if (SameValue(ToString(n), argument)) { return n; }
+	return void 0;
+};