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-strategies/index.d.ts

@@ -0,0 +1,20 @@
+import { CacheFirst } from './CacheFirst.js';
+import { CacheOnly } from './CacheOnly.js';
+import { NetworkFirst, NetworkFirstOptions } from './NetworkFirst.js';
+import { NetworkOnly, NetworkOnlyOptions } from './NetworkOnly.js';
+import { StaleWhileRevalidate } from './StaleWhileRevalidate.js';
+import { Strategy, StrategyOptions } from './Strategy.js';
+import { StrategyHandler } from './StrategyHandler.js';
+import './_version.js';
+declare global {
+    interface FetchEvent {
+        readonly preloadResponse: Promise<any>;
+    }
+}
+/**
+ * There are common caching strategies that most service workers will need
+ * and use. This module provides simple implementations of these strategies.
+ *
+ * @module workbox-strategies
+ */
+export { CacheFirst, CacheOnly, NetworkFirst, NetworkFirstOptions, NetworkOnly, NetworkOnlyOptions, StaleWhileRevalidate, Strategy, StrategyHandler, StrategyOptions, };