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/source-map/lib/url.js

@@ -0,0 +1,13 @@
+/* -*- Mode: js; js-indent-level: 2; -*- */
+/*
+ * Copyright 2011 Mozilla Foundation and contributors
+ * Licensed under the New BSD license. See LICENSE or:
+ * http://opensource.org/licenses/BSD-3-Clause
+ */
+"use strict";
+
+// Note: This file is overridden in the 'package.json#browser' field to
+// substitute lib/url-browser.js instead.
+
+// Use the URL global for Node 10, and the 'url' module for Node 8.
+module.exports = typeof URL === "function" ? URL : require("url").URL;