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/AsyncFromSyncIteratorContinuation.js

@@ -0,0 +1,45 @@
+'use strict';
+
+var GetIntrinsic = require('get-intrinsic');
+
+var $SyntaxError = require('es-errors/syntax');
+var $TypeError = require('es-errors/type');
+var isObject = require('es-object-atoms/isObject');
+var $Promise = GetIntrinsic('%Promise%', true);
+
+var callBound = require('call-bound');
+
+var CreateIteratorResultObject = require('./CreateIteratorResultObject');
+var IteratorComplete = require('./IteratorComplete');
+var IteratorValue = require('./IteratorValue');
+var PromiseResolve = require('./PromiseResolve');
+
+var $then = callBound('Promise.prototype.then', true);
+
+// https://262.ecma-international.org/16.0/#sec-asyncfromsynciteratorcontinuation
+
+module.exports = function AsyncFromSyncIteratorContinuation(result) {
+	if (!isObject(result)) {
+		throw new $TypeError('Assertion failed: Type(O) is not Object');
+	}
+
+	if (arguments.length > 1) {
+		throw new $SyntaxError('although AsyncFromSyncIteratorContinuation should take a second argument, it is not used in this implementation');
+	}
+
+	if (!$Promise) {
+		throw new $SyntaxError('This environment does not support Promises.');
+	}
+
+	return new $Promise(function (resolve) {
+		var done = IteratorComplete(result); // step 2
+		var value = IteratorValue(result); // step 4
+		var valueWrapper = PromiseResolve($Promise, value); // step 6
+
+		// eslint-disable-next-line no-shadow
+		var onFulfilled = function (value) { // steps 8-9
+			return CreateIteratorResultObject(value, done); // step 8.a
+		};
+		resolve($then(valueWrapper, onFulfilled)); // step 11
+	}); // step 12
+};