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/2020/IsNoTearConfiguration.js

@@ -0,0 +1,16 @@
+'use strict';
+
+var IsUnclampedIntegerElementType = require('./IsUnclampedIntegerElementType');
+var IsBigIntElementType = require('./IsBigIntElementType');
+
+// https://262.ecma-international.org/11.0/#sec-isnotearconfiguration
+
+module.exports = function IsNoTearConfiguration(type, order) {
+	if (IsUnclampedIntegerElementType(type)) {
+		return true;
+	}
+	if (IsBigIntElementType(type) && order !== 'Init' && order !== 'Unordered') {
+		return true;
+	}
+	return false;
+};