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/index.d.ts

@@ -0,0 +1,10 @@
+import { ErrorObject } from 'ajv';
+import { ValidationError } from './types/ValidationError';
+export interface BetterAjvErrorsOptions<S = any> {
+    errors: ErrorObject[] | null | undefined;
+    data: any;
+    schema: S;
+    basePath?: string;
+}
+export declare const betterAjvErrors: <S = any>({ errors, data, schema, basePath, }: BetterAjvErrorsOptions<S>) => ValidationError[];
+export { ValidationError };