fix(canvas/WorkspaceNode.tsx): add missing useMemo import

CI failure: "Cannot find name 'useMemo'" at line 363.
useMemo was called but not imported — likely dropped during refactor.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

canvas/src/components/WorkspaceNode.tsx

@@ -1,6 +1,6 @@
 "use client";
 
-import { useCallback } from "react";
+import { useCallback, useMemo } from "react";
 import { Handle, NodeResizer, Position, type NodeProps, type Node } from "@xyflow/react";
 import { useCanvasStore, type WorkspaceNodeData } from "@/store/canvas";
 import { showToast } from "@/components/Toaster";