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/@apideck/better-ajv-errors/dist/lib/utils.d.ts

@@ -0,0 +1,8 @@
+export declare const pointerToDotNotation: (pointer: string) => string;
+export declare const cleanAjvMessage: (message: string) => string;
+export declare const getLastSegment: (path: string) => string;
+export declare const safeJsonPointer: <T>({ object, pnter, fallback }: {
+    object: any;
+    pnter: string;
+    fallback: T;
+}) => T;