fix: clear structured log viewer rows on stack switch (#1448)

The structured log viewer accumulated log rows across stack switches
because the useEffect cleanup closed the old WebSocket but never
cleared the committed rows state. Reset rows, row IDs, and auto-follow
at the top of the effect before connecting to the new stack. The level
filter is intentionally preserved across switches.

Closes #1444

frontend/src/components/StructuredLogViewer.tsx

@@ -68,6 +68,13 @@ export default function StructuredLogViewer({ stackName }: StructuredLogViewerPr
   useEffect(() => { followingRef.current = following; }, [following]);
 
   useEffect(() => {
+    // Reset state accumulated from the previous stack before connecting.
+    // eslint-disable-next-line react-hooks/set-state-in-effect
+    setRows([]);
+    rowIdRef.current = 0;
+    setFollowing(true);
+    followingRef.current = true;
+
     const cleanStackName = stackName.replace(/\.(yml|yaml)$/, '');
     const wsProtocol = window.location.protocol === 'https:' ? 'wss:' : 'ws:';
     const activeNodeId = localStorage.getItem('sencho-active-node') || '';