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/workbox-routing/index.js

@@ -0,0 +1,19 @@
+/*
+  Copyright 2018 Google LLC
+
+  Use of this source code is governed by an MIT-style
+  license that can be found in the LICENSE file or at
+  https://opensource.org/licenses/MIT.
+*/
+import { NavigationRoute, } from './NavigationRoute.js';
+import { RegExpRoute } from './RegExpRoute.js';
+import { registerRoute } from './registerRoute.js';
+import { Route } from './Route.js';
+import { Router } from './Router.js';
+import { setCatchHandler } from './setCatchHandler.js';
+import { setDefaultHandler } from './setDefaultHandler.js';
+import './_version.js';
+/**
+ * @module workbox-routing
+ */
+export { NavigationRoute, RegExpRoute, registerRoute, Route, Router, setCatchHandler, setDefaultHandler, };