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/safe-array-concat/index.d.ts

@@ -0,0 +1,6 @@
+declare function safeArrayConcat<T = unknown>(
+    item: T | readonly T[],
+    ...items: (T | readonly T[])[],
+): T[];
+
+export = safeArrayConcat;