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/NumberToString.js

@@ -0,0 +1,17 @@
+'use strict';
+
+var GetIntrinsic = require('get-intrinsic');
+
+var $String = GetIntrinsic('%String%');
+var $TypeError = require('es-errors/type');
+
+// https://262.ecma-international.org/9.0/#sec-tostring-applied-to-the-number-type
+
+module.exports = function NumberToString(m) {
+	if (typeof m !== 'number') {
+		throw new $TypeError('Assertion failed: "m" must be a String');
+	}
+
+	return $String(m);
+};
+