fix(nfs): reconcile clients[i].advanced type mismatch (object vs string)

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.

web/src/api.ts

@@ -15,7 +15,16 @@ export interface NFSClient {
   async: boolean;
   root_squash: boolean;
   subtree_check: boolean;
-  advanced: string;
+  advanced: NFSAdvanced;
+}
+
+export interface NFSAdvanced {
+  all_squash: boolean;
+  secure: boolean;
+  wdelay: boolean;
+  hide: boolean;
+  crossmnt: boolean;
+  [key: string]: boolean;
 }
 
 export interface NFSExport {