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/NewPromiseCapability.js

@@ -0,0 +1,34 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+
+var IsCallable = require('./IsCallable');
+var IsConstructor = require('./IsConstructor');
+
+// https://262.ecma-international.org/6.0/#sec-newpromisecapability
+
+module.exports = function NewPromiseCapability(C) {
+	if (!IsConstructor(C)) {
+		throw new $TypeError('C must be a constructor'); // step 1
+	}
+
+	var resolvingFunctions = { '[[Resolve]]': void undefined, '[[Reject]]': void undefined }; // step 3
+
+	var promise = new C(function (resolve, reject) { // steps 4-5
+		if (typeof resolvingFunctions['[[Resolve]]'] !== 'undefined' || typeof resolvingFunctions['[[Reject]]'] !== 'undefined') {
+			throw new $TypeError('executor has already been called'); // step 4.a, 4.b
+		}
+		resolvingFunctions['[[Resolve]]'] = resolve; // step 4.c
+		resolvingFunctions['[[Reject]]'] = reject; // step 4.d
+	}); // step 4-6
+
+	if (!IsCallable(resolvingFunctions['[[Resolve]]']) || !IsCallable(resolvingFunctions['[[Reject]]'])) {
+		throw new $TypeError('executor must provide valid resolve and reject functions'); // steps 7-8
+	}
+
+	return {
+		'[[Promise]]': promise,
+		'[[Resolve]]': resolvingFunctions['[[Resolve]]'],
+		'[[Reject]]': resolvingFunctions['[[Reject]]']
+	}; // step 9
+};