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/compile/validate/boolSchema.d.ts

@@ -0,0 +1,4 @@
+import type { SchemaCxt } from "..";
+import { Name } from "../codegen";
+export declare function topBoolOrEmptySchema(it: SchemaCxt): void;
+export declare function boolOrEmptySchema(it: SchemaCxt, valid: Name): void;