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-recipes/index.js

@@ -0,0 +1,18 @@
+/*
+  Copyright 2020 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 { googleFontsCache } from './googleFontsCache';
+import { imageCache } from './imageCache';
+import { staticResourceCache, } from './staticResourceCache';
+import { pageCache } from './pageCache';
+import { offlineFallback } from './offlineFallback';
+import { warmStrategyCache } from './warmStrategyCache';
+import './_version.js';
+/**
+ * @module workbox-recipes
+ */
+export { googleFontsCache, imageCache, offlineFallback, pageCache, staticResourceCache, warmStrategyCache, };