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/nullable.ts

@@ -0,0 +1,21 @@
+import type {KeywordCxt} from "../../compile/validate"
+import {_, not, nil, Code, Name} from "../../compile/codegen"
+
+export function checkNullable(
+  {gen, data, parentSchema}: KeywordCxt,
+  cond: Code = nil
+): [Name, Code] {
+  const valid = gen.name("valid")
+  if (parentSchema.nullable) {
+    gen.let(valid, _`${data} === null`)
+    cond = not(valid)
+  } else {
+    gen.let(valid, false)
+  }
+  return [valid, cond]
+}
+
+export function checkNullableObject(cxt: KeywordCxt, cond: Code): [Name, Code] {
+  const [valid, cond_] = checkNullable(cxt, cond)
+  return [valid, _`${cond_} && typeof ${cxt.data} == "object" && !Array.isArray(${cxt.data})`]
+}