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/additional-manifest-entries-transform.d.ts

@@ -0,0 +1,13 @@
+import { ManifestEntry } from '../types';
+type AdditionalManifestEntriesTransform = {
+    (manifest: Array<ManifestEntry & {
+        size: number;
+    }>): {
+        manifest: Array<ManifestEntry & {
+            size: number;
+        }>;
+        warnings: string[];
+    };
+};
+export declare function additionalManifestEntriesTransform(additionalManifestEntries: Array<ManifestEntry | string>): AdditionalManifestEntriesTransform;
+export {};