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/jtd/error.d.ts

@@ -0,0 +1,9 @@
+import type { KeywordErrorDefinition, KeywordErrorCxt, ErrorObject } from "../../types";
+import { Code } from "../../compile/codegen";
+export type _JTDTypeError<K extends string, T extends string, S> = ErrorObject<K, {
+    type: T;
+    nullable: boolean;
+}, S>;
+export declare function typeError(t: string): KeywordErrorDefinition;
+export declare function typeErrorMessage({ parentSchema }: KeywordErrorCxt, t: string): string;
+export declare function typeErrorParams({ parentSchema }: KeywordErrorCxt, t: string): Code;