fix(canvas): replace undefined structureSignature with nodeSetSignature

The fitView effect referenced structureSignature which does not exist
in this scope — the correct variable is nodeSetSignature. This caused
a ReferenceError at runtime when the effect ran after adding an edge.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

src/pages/workflows/components/WorkflowCanvas.tsx

@@ -164,7 +164,7 @@ export function WorkflowCanvas({
       flowInstance.fitView({ padding: 0.3, duration: 260, maxZoom: 1.05, minZoom: 0.2 })
     }, 30)
     return () => window.clearTimeout(t)
-  }, [flowInstance, structureSignature])
+  }, [flowInstance, nodeSetSignature])
 
   // Edge creation with validation
   const handleConnect = useCallback((conn: Connection) => {