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/2017/IterableToList.js

@@ -0,0 +1,21 @@
+'use strict';
+
+var GetIterator = require('./GetIterator');
+var IteratorStep = require('./IteratorStep');
+var IteratorValue = require('./IteratorValue');
+
+// https://262.ecma-international.org/8.0/#sec-iterabletolist
+
+module.exports = function IterableToList(items, method) {
+	var iterator = GetIterator(items, method);
+	var values = [];
+	var next = true;
+	while (next) {
+		next = IteratorStep(iterator);
+		if (next) {
+			var nextValue = IteratorValue(next);
+			values[values.length] = nextValue;
+		}
+	}
+	return values;
+};