feat: add app preview via NodePort

- Backend: getPreviewInfo() in KubernetesService auto-patches ClusterIP
  service to NodePort for direct external access
- Backend: GET /applications/:id/preview endpoint returns access URL
  with nodePort, host IP (extracted from kubeconfig), and ingress URL
- Frontend: '🌐 Preview' button on app detail page (visible when running)
  opens the deployed app in a new browser tab via NodePort URL
- Tested: service patched to NodePort 30107 successfully

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

@@ -176,6 +176,16 @@ export default function AppDetailPage() {
     onError: () => toast.error('Failed to update resources'),
   });
 
+  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}`);
+    },
+    onError: () => toast.error('Failed to get preview URL. Make sure the app is deployed.'),
+  });
+
   const uploadMutation = useMutation({
     mutationFn: (file: File) => {
       const formData = new FormData();
@@ -314,6 +324,18 @@ export default function AppDetailPage() {
                   {redeployMutation.isPending ? '⏳ Rebuilding...' : '🔄 Redeploy'}
                 </button>
               )}
+
+              {/* Preview: open the running app in a new tab */}
+              {isRunning && (
+                <button
+                  onClick={() => previewMutation.mutate()}
+                  disabled={previewMutation.isPending}
+                  className="px-4 py-2 rounded-lg text-sm font-medium bg-emerald-50 text-emerald-700 hover:bg-emerald-100 border border-emerald-200 disabled:opacity-50 transition-colors"
+                  title="Open the running application in a new browser tab"
+                >
+                  {previewMutation.isPending ? '⏳ Loading...' : '🌐 Preview'}
+                </button>
+              )}
             </>
           )}