feat: add LoadingOverlay component with contextual busy messages for async operations across cluster sync, firewall preview/apply, IPAM discovery/export, node agent installer, and policy compilation

Add LoadingOverlay component with spinner, customizable title/message, and backdrop blur styling, implement busy state tracking with setBusyMessage in Clusters/FirewallPreview/Ipam/Nodes/Policies pages, wrap async operations (cluster test/sync, firewall preview/apply, IPAM discover/export CSV, node agent installer

frontend/src/pages/Nodes.tsx

@@ -5,6 +5,7 @@ import { useState } from "react";
 import { AgentInstallInfo, api, Node } from "../api/client";
 import { DataTable } from "../components/DataTable";
 import { iconButtonClass, secondaryButtonClass } from "../components/FormControls";
+import { LoadingOverlay } from "../components/LoadingOverlay";
 import { Modal } from "../components/Modal";
 import { PageHeader } from "../components/PageHeader";
 
@@ -47,6 +48,7 @@ export function Nodes() {
   return (
     <>
       <PageHeader title="Nodes" subtitle="Cluster nodes, capacity, health, and NexaFabric agent enrollment." />
+      <LoadingOverlay open={installInfo.isPending} message="Generating node agent installer..." />
       {nodes.isLoading ? <div className="rounded-md border border-border bg-panel p-8 text-sm">Loading...</div> : null}
       {nodes.error ? <div className="rounded-md border border-danger p-4 text-sm text-danger">Failed to load nodes.</div> : null}
       {!nodes.isLoading && !nodes.error ? (