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-precaching/utils/createCacheKey.d.ts

@@ -0,0 +1,17 @@
+import { PrecacheEntry } from '../_types.js';
+import '../_version.js';
+interface CacheKey {
+    cacheKey: string;
+    url: string;
+}
+/**
+ * Converts a manifest entry into a versioned URL suitable for precaching.
+ *
+ * @param {Object|string} entry
+ * @return {string} A URL with versioning info.
+ *
+ * @private
+ * @memberof workbox-precaching
+ */
+export declare function createCacheKey(entry: PrecacheEntry | string): CacheKey;
+export {};