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-sw/index.mjs

@@ -0,0 +1,17 @@
+/*
+  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 {WorkboxSW} from './controllers/WorkboxSW.mjs';
+import './_version.mjs';
+
+/**
+ * @namespace workbox
+ */
+
+// Don't export anything, just expose a global.
+self.workbox = new WorkboxSW();