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/draft7.ts

@@ -0,0 +1,17 @@
+import type {Vocabulary} from "../types"
+import coreVocabulary from "./core"
+import validationVocabulary from "./validation"
+import getApplicatorVocabulary from "./applicator"
+import formatVocabulary from "./format"
+import {metadataVocabulary, contentVocabulary} from "./metadata"
+
+const draft7Vocabularies: Vocabulary[] = [
+  coreVocabulary,
+  validationVocabulary,
+  getApplicatorVocabulary(),
+  formatVocabulary,
+  metadataVocabulary,
+  contentVocabulary,
+]
+
+export default draft7Vocabularies