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/index.d.ts

@@ -0,0 +1,10 @@
+import { copyWorkboxLibraries } from './lib/copy-workbox-libraries';
+import { getModuleURL } from './lib/cdn-utils';
+import { generateSW } from './generate-sw';
+import { getManifest } from './get-manifest';
+import { injectManifest } from './inject-manifest';
+/**
+ * @module workbox-build
+ */
+export { copyWorkboxLibraries, generateSW, getManifest, getModuleURL, injectManifest, };
+export * from './types';