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/applicator/prefixItems.ts

@@ -0,0 +1,12 @@
+import type {CodeKeywordDefinition} from "../../types"
+import {validateTuple} from "./items"
+
+const def: CodeKeywordDefinition = {
+  keyword: "prefixItems",
+  type: "array",
+  schemaType: ["array"],
+  before: "uniqueItems",
+  code: (cxt) => validateTuple(cxt, "items"),
+}
+
+export default def