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-get/Get.js

@@ -0,0 +1,26 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+
+var inspect = require('object-inspect');
+
+var isObject = require('es-object-atoms/isObject');
+
+var isPropertyKey = require('./isPropertyKey');
+
+// https://262.ecma-international.org/6.0/#sec-get-o-p
+
+/** @type {import('./Get')} */
+module.exports = function Get(O, P) {
+	// 7.3.1.1
+	var isObj = isObject(O);
+	if (!isObj) {
+		throw new $TypeError('Assertion failed: Type(O) is not Object');
+	}
+	// 7.3.1.2
+	if (!isPropertyKey(P)) {
+		throw new $TypeError('Assertion failed: P is not a Property Key, got ' + inspect(P));
+	}
+	// 7.3.1.3
+	return O[P];
+};