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/lib/vocabularies/errors.ts

@@ -0,0 +1,18 @@
+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