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/es-abstract-get/Get.d.ts

@@ -0,0 +1,11 @@
+import { PropertyKey } from './isPropertyKey';
+
+declare function Get<
+    T extends object,
+    K extends keyof T & PropertyKey,
+>(
+    O: T,
+    P: K,
+): T[keyof T] | undefined;
+
+export = Get;