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/validation/dependentRequired.ts

@@ -0,0 +1,23 @@
+import type {CodeKeywordDefinition, ErrorObject} from "../../types"
+import {
+  validatePropertyDeps,
+  error,
+  DependenciesErrorParams,
+  PropertyDependencies,
+} from "../applicator/dependencies"
+
+export type DependentRequiredError = ErrorObject<
+  "dependentRequired",
+  DependenciesErrorParams,
+  PropertyDependencies
+>
+
+const def: CodeKeywordDefinition = {
+  keyword: "dependentRequired",
+  type: "object",
+  schemaType: "object",
+  error,
+  code: (cxt) => validatePropertyDeps(cxt),
+}
+
+export default def