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-recipes/pageCache.d.ts

@@ -0,0 +1,23 @@
+import { RouteMatchCallback, WorkboxPlugin } from 'workbox-core/types.js';
+import './_version.js';
+export interface PageCacheOptions {
+    cacheName?: string;
+    matchCallback?: RouteMatchCallback;
+    networkTimeoutSeconds?: number;
+    plugins?: Array<WorkboxPlugin>;
+    warmCache?: Array<string>;
+}
+/**
+ * An implementation of a page caching recipe with a network timeout
+ *
+ * @memberof workbox-recipes
+ *
+ * @param {Object} [options]
+ * @param {string} [options.cacheName] Name for cache. Defaults to pages
+ * @param {RouteMatchCallback} [options.matchCallback] Workbox callback function to call to match to. Defaults to request.mode === 'navigate';
+ * @param {number} [options.networkTimoutSeconds] Maximum amount of time, in seconds, to wait on the network before falling back to cache. Defaults to 3
+ * @param {WorkboxPlugin[]} [options.plugins] Additional plugins to use for this recipe
+ * @param {string[]} [options.warmCache] Paths to call to use to warm this cache
+ */
+declare function pageCache(options?: PageCacheOptions): void;
+export { pageCache };