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/2025/GetPromiseResolve.js

@@ -0,0 +1,20 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+
+var Get = require('./Get');
+var IsCallable = require('./IsCallable');
+var IsConstructor = require('./IsConstructor');
+
+// https://262.ecma-international.org/12.0/#sec-getpromiseresolve
+
+module.exports = function GetPromiseResolve(promiseConstructor) {
+	if (!IsConstructor(promiseConstructor)) {
+		throw new $TypeError('Assertion failed: `promiseConstructor` must be a constructor');
+	}
+	var promiseResolve = Get(promiseConstructor, 'resolve');
+	if (IsCallable(promiseResolve) === false) {
+		throw new $TypeError('`resolve` method is not callable');
+	}
+	return promiseResolve;
+};