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/GetIteratorFlattenable.js

@@ -0,0 +1,48 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+var isObject = require('es-object-atoms/isObject');
+
+var AdvanceStringIndex = require('./AdvanceStringIndex');
+var Call = require('./Call');
+var GetIteratorDirect = require('./GetIteratorDirect');
+var GetMethod = require('./GetMethod');
+var IsArray = require('./IsArray');
+
+var getIteratorMethod = require('../helpers/getIteratorMethod');
+
+// https://262.ecma-international.org/16.0/#sec-getiteratorflattenable
+
+module.exports = function GetIteratorFlattenable(obj, primitiveHandling) {
+	if (primitiveHandling !== 'REJECT-PRIMITIVES' && primitiveHandling !== 'ITERATE-STRING-PRIMITIVES') {
+		throw new $TypeError('Assertion failed: `stringHandling` must be "REJECT-PRIMITIVES" or "ITERATE-STRING-PRIMITIVES"');
+	}
+
+	if (!isObject(obj)) {
+		if (primitiveHandling === 'REJECT-PRIMITIVES' || typeof obj !== 'string') {
+			throw new $TypeError('obj must be an Object'); // step 1.a
+		}
+	}
+
+	// var method = GetMethod(obj, Symbol.iterator); // step 2
+	var method = getIteratorMethod(
+		{
+			AdvanceStringIndex: AdvanceStringIndex,
+			GetMethod: GetMethod,
+			IsArray: IsArray
+		},
+		obj
+	);
+
+	var iterator;
+	if (typeof method === 'undefined') { // step 3
+		iterator = obj; // step 3.a
+	} else { // step 4
+		iterator = Call(method, obj); // step 4.a
+	}
+
+	if (!isObject(iterator)) {
+		throw new $TypeError('iterator must be an Object'); // step 5
+	}
+	return GetIteratorDirect(iterator); // step 6
+};