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/runtime/uri.ts

@@ -0,0 +1,6 @@
+import * as uri from "fast-uri"
+
+type URI = typeof uri & {code: string}
+;(uri as URI).code = 'require("ajv/dist/runtime/uri").default'
+
+export default uri as URI