fix(workflow): reinitialize mounted ref on effect setup for StrictMode

In React StrictMode (dev mode), effects are run twice to help detect
side effects. The cleanup-only pattern left isMountedRef as false after
StrictMode's simulated unmount-remount cycle, causing stop/cancel
operations to be skipped even when the component was mounted.

Now the effect setup explicitly sets isMountedRef.current = true,
ensuring correct behavior in both development and production.

web/app/components/workflow/panel/debug-and-preview/hooks/use-chat-flow-control.ts

@@ -21,8 +21,11 @@ export function useChatFlowControl({
   const invalidateRun = useStore(s => s.invalidateRun)
 
   const isMountedRef = useRef(true)
-  useEffect(() => () => {
-    isMountedRef.current = false
+  useEffect(() => {
+    isMountedRef.current = true
+    return () => {
+      isMountedRef.current = false
+    }
   }, [])
 
   const { handleNodeCancelRunningStatus } = useNodesInteractionsWithoutSync(isMountedRef)