Use per-deploy preview hosts under the site root domain.

Build hosts as <userPrefix>-<deploymentNumber>-preview.<rootDomain> from FRONTEND_URL, wire them through ingress/TLS, and open them from the preview API.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/app/dashboard/apps/[id]/page.tsx

@@ -867,9 +867,13 @@ export default function AppDetailPage() {
   const previewMutation = useMutation({
     mutationFn: () => api.get(`/applications/${appId}/preview`).then((r) => r.data),
     onSuccess: (data: { url: string; nodePort: number; host: string; ingressUrl?: string }) => {
-      // Open the preview URL in a new tab
-      window.open(data.url, '_blank');
-      toast.success(`Preview opened on port ${data.nodePort}`);
+      const targetUrl = data.ingressUrl || data.url;
+      window.open(targetUrl, '_blank');
+      if (data.ingressUrl) {
+        toast.success('Preview opened on HTTPS preview domain.');
+      } else {
+        toast.success(`Preview opened on port ${data.nodePort}`);
+      }
     },
     onError: () => toast.error('Failed to get preview URL. Make sure the app is deployed.'),
   });