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-streams/index.d.ts

@@ -0,0 +1,10 @@
+import { concatenate } from './concatenate.js';
+import { concatenateToResponse } from './concatenateToResponse.js';
+import { isSupported } from './isSupported.js';
+import { strategy, StreamsHandlerCallback } from './strategy.js';
+import './_version.js';
+/**
+ * @module workbox-streams
+ */
+export { concatenate, concatenateToResponse, isSupported, strategy, StreamsHandlerCallback, };
+export * from './_types.js';