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/dynamic/recursiveAnchor.ts

@@ -0,0 +1,14 @@
+import type {CodeKeywordDefinition} from "../../types"
+import {dynamicAnchor} from "./dynamicAnchor"
+import {checkStrictMode} from "../../compile/util"
+
+const def: CodeKeywordDefinition = {
+  keyword: "$recursiveAnchor",
+  schemaType: "boolean",
+  code(cxt) {
+    if (cxt.schema) dynamicAnchor(cxt, "")
+    else checkStrictMode(cxt.it, "$recursiveAnchor: false is ignored")
+  },
+}
+
+export default def