backend: db.NFSClient.Advanced is db.NFSAdvanced (struct → JSON object)
handler: nfsClientRequest.Advanced was string → now db.NFSAdvanced
frontend: api.ts NFSClient.advanced string → NFSAdvanced (object)
frontend: Nfs.tsx updated to work with object advanced per host
Without this fix, editing an existing NFS export with per-host options
(loaded from DB via migration 0005) would 400 on PUT because the
backend expected a string but received a JSON object.