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-build/build/cdn-details.json

@@ -0,0 +1,6 @@
+{
+    "origin": "https://storage.googleapis.com",
+    "bucketName": "workbox-cdn",
+    "releasesDir": "releases",
+    "latestVersion": "7.4.1"
+}