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/errors.d.ts

@@ -0,0 +1,9 @@
+import type { TypeError } from "../compile/validate/dataType";
+import type { ApplicatorKeywordError } from "./applicator";
+import type { ValidationKeywordError } from "./validation";
+import type { FormatError } from "./format/format";
+import type { UnevaluatedPropertiesError } from "./unevaluated/unevaluatedProperties";
+import type { UnevaluatedItemsError } from "./unevaluated/unevaluatedItems";
+import type { DependentRequiredError } from "./validation/dependentRequired";
+import type { DiscriminatorError } from "./discriminator";
+export type DefinedError = TypeError | ApplicatorKeywordError | ValidationKeywordError | FormatError | UnevaluatedPropertiesError | UnevaluatedItemsError | DependentRequiredError | DiscriminatorError;