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/next.ts

@@ -0,0 +1,8 @@
+import type {Vocabulary} from "../types"
+import dependentRequired from "./validation/dependentRequired"
+import dependentSchemas from "./applicator/dependentSchemas"
+import limitContains from "./validation/limitContains"
+
+const next: Vocabulary = [dependentRequired, dependentSchemas, limitContains]
+
+export default next