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/2022/StringToCodePoints.js

@@ -0,0 +1,22 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+
+var CodePointAt = require('./CodePointAt');
+
+// https://262.ecma-international.org/12.0/#sec-stringtocodepoints
+
+module.exports = function StringToCodePoints(string) {
+	if (typeof string !== 'string') {
+		throw new $TypeError('Assertion failed: `string` must be a String');
+	}
+	var codePoints = [];
+	var size = string.length;
+	var position = 0;
+	while (position < size) {
+		var cp = CodePointAt(string, position);
+		codePoints[codePoints.length] = cp['[[CodePoint]]'];
+		position += cp['[[CodeUnitCount]]'];
+	}
+	return codePoints;
+};