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/lib/write-sw-using-default-template.d.ts

@@ -0,0 +1,4 @@
+import { GenerateSWOptions, ManifestEntry } from '../types';
+export declare function writeSWUsingDefaultTemplate({ babelPresetEnvTargets, cacheId, cleanupOutdatedCaches, clientsClaim, directoryIndex, disableDevLogs, ignoreURLParametersMatching, importScripts, inlineWorkboxRuntime, manifestEntries, mode, navigateFallback, navigateFallbackDenylist, navigateFallbackAllowlist, navigationPreload, offlineGoogleAnalytics, runtimeCaching, skipWaiting, sourcemap, swDest, }: GenerateSWOptions & {
+    manifestEntries: Array<ManifestEntry>;
+}): Promise<Array<string>>;