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/@rollup/plugin-terser/src/index.ts

@@ -0,0 +1,8 @@
+import { runWorker } from './worker';
+import terser from './module';
+
+runWorker();
+
+export * from './type';
+
+export default terser;