fix: restore connection guard on publish section in DataPanel

The publish UI was showing even when disconnected because the
client null check was dropped during migration. Replaced with
isConnected store check.

Author: bburda

src/components/DataPanel.tsx

@@ -8,6 +8,7 @@ import { JsonFormViewer } from '@/components/JsonFormViewer';
 import { TopicPublishForm } from '@/components/TopicPublishForm';
 import type { ComponentTopic, TopicEndpoint, QosProfile, SovdResourceEntityType } from '@/lib/types';
 import { cn } from '@/lib/utils';
+import { useAppStore } from '@/lib/store';
 
 interface DataPanelProps {
     /** Data item from the API */
@@ -198,8 +199,9 @@ export function DataPanel({
 }: DataPanelProps) {
     const [publishValue, setPublishValue] = useState<unknown>(topic.type_info?.default_value || topic.data || {});
 
+    const isConnected = useAppStore((state) => state.isConnected);
     const hasData = topic.status === 'data' && topic.data !== null && topic.data !== undefined;
-    const canPublish = !!(topic.type || topic.type_info || topic.data);
+    const canPublish = isConnected && !!(topic.type || topic.type_info || topic.data);
 
     const handleCopyFromLast = () => {
         if (topic.data) {