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/resolve/SECURITY.md

@@ -0,0 +1,13 @@
+# Security
+
+Please [file a private vulnerability report](https://github.com/browserify/resolve/security/advisories/new),
+or email [@ljharb](https://github.com/ljharb),
+if you have a potential security vulnerability to report.
+
+## Incident Response
+
+See our [Incident Response Process](.github/INCIDENT_RESPONSE_PROCESS.md).
+
+## Threat Model
+
+See [THREAT_MODEL.md](./THREAT_MODEL.md).