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/2023/CodePointsToString.js

@@ -0,0 +1,25 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+
+var UTF16EncodeCodePoint = require('./UTF16EncodeCodePoint');
+var IsArray = require('./IsArray');
+
+var forEach = require('../helpers/forEach');
+var isCodePoint = require('../helpers/isCodePoint');
+
+// https://262.ecma-international.org/12.0/#sec-codepointstostring
+
+module.exports = function CodePointsToString(text) {
+	if (!IsArray(text)) {
+		throw new $TypeError('Assertion failed: `text` must be a sequence of Unicode Code Points');
+	}
+	var result = '';
+	forEach(text, function (cp) {
+		if (!isCodePoint(cp)) {
+			throw new $TypeError('Assertion failed: `text` must be a sequence of Unicode Code Points');
+		}
+		result += UTF16EncodeCodePoint(cp);
+	});
+	return result;
+};