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/compile/ref_error.ts

@@ -0,0 +1,13 @@
+import {resolveUrl, normalizeId, getFullPath} from "./resolve"
+import type {UriResolver} from "../types"
+
+export default class MissingRefError extends Error {
+  readonly missingRef: string
+  readonly missingSchema: string
+
+  constructor(resolver: UriResolver, baseId: string, ref: string, msg?: string) {
+    super(msg || `can't resolve reference ${ref} from id ${baseId}`)
+    this.missingRef = resolveUrl(resolver, baseId, ref)
+    this.missingSchema = normalizeId(getFullPath(resolver, this.missingRef))
+  }
+}