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/ajv/dist/vocabularies/dynamic/dynamicRef.d.ts

@@ -0,0 +1,5 @@
+import type { CodeKeywordDefinition } from "../../types";
+import type { KeywordCxt } from "../../compile/validate";
+declare const def: CodeKeywordDefinition;
+export declare function dynamicRef(cxt: KeywordCxt, ref: string): void;
+export default def;