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/helpers/assign.js

@@ -0,0 +1,22 @@
+'use strict';
+
+var GetIntrinsic = require('get-intrinsic');
+
+var hasOwn = require('hasown');
+
+var $assign = GetIntrinsic('%Object.assign%', true);
+
+module.exports = function assign(target, source) {
+	if ($assign) {
+		return $assign(target, source);
+	}
+
+	// eslint-disable-next-line no-restricted-syntax
+	for (var key in source) {
+		if (hasOwn(source, key)) {
+			// eslint-disable-next-line no-param-reassign
+			target[key] = source[key];
+		}
+	}
+	return target;
+};