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/jtd/error.ts

@@ -0,0 +1,23 @@
+import type {KeywordErrorDefinition, KeywordErrorCxt, ErrorObject} from "../../types"
+import {_, Code} from "../../compile/codegen"
+
+export type _JTDTypeError<K extends string, T extends string, S> = ErrorObject<
+  K,
+  {type: T; nullable: boolean},
+  S
+>
+
+export function typeError(t: string): KeywordErrorDefinition {
+  return {
+    message: (cxt) => typeErrorMessage(cxt, t),
+    params: (cxt) => typeErrorParams(cxt, t),
+  }
+}
+
+export function typeErrorMessage({parentSchema}: KeywordErrorCxt, t: string): string {
+  return parentSchema?.nullable ? `must be ${t} or null` : `must be ${t}`
+}
+
+export function typeErrorParams({parentSchema}: KeywordErrorCxt, t: string): Code {
+  return _`{type: ${t}, nullable: ${!!parentSchema?.nullable}}`
+}