Add subscription validation and first-run onboarding reveal

src/web/App.jsx

@@ -5,6 +5,7 @@ import { api } from './api.js';
 import { ClientOverviewPage } from './components/ClientOverviewPage.jsx';
 
 function App() {
+  const previewReady = new URLSearchParams(window.location.search).has('preview-ready');
   const [state, setState] = useState(null);
   const [subscriptionUrl, setSubscriptionUrl] = useState('');
   const [servers, setServers] = useState([]);
@@ -86,13 +87,13 @@ function App() {
       <div className="app-body client-mode">
         <main className="app-main">
           <ClientOverviewPage
-            state={state}
+            state={previewReady ? { ...state, mode: 'client', hasSubscription: true, subscriptionHost: 'harbor.example', selectedTag: 'Amsterdam', proxyPort: 8082 } : state}
             busy={busy}
             error={error}
             subscriptionUrl={subscriptionUrl}
             setSubscriptionUrl={setSubscriptionUrl}
-            servers={servers}
-            pendingTag={pendingTag}
+            servers={previewReady ? [{ tag: 'Amsterdam', server: '127.0.0.1', server_port: 443 }] : servers}
+            pendingTag={previewReady ? 'Amsterdam' : pendingTag}
             setPendingTag={setPendingTag}
             onFetchSubscription={fetchSubscription}
             onRefreshSubscription={refreshSubscription}