Adopt shared WebUI structural primitives

webui/src/features/dataflow/DataflowCanvas.tsx

@@ -14,6 +14,7 @@ import {
   type Edge,
   type ReactFlowInstance
 } from "@xyflow/react";
+import { StatePanel } from "@govoplan/core-webui";
 import { definitionConnectionError } from "@govoplan/core-webui/definition-graph";
 import type {
   DataflowDiagnostic,
@@ -226,7 +227,7 @@ export default function DataflowCanvas({
   return (
     <div
       ref={canvasRef}
-      className="dataflow-canvas"
+      className="definition-graph-canvas dataflow-canvas"
       tabIndex={0}
       aria-label="Dataflow graph canvas"
       onDragOver={(event) => {
@@ -360,7 +361,7 @@ export default function DataflowCanvas({
         <Controls showInteractive={false} />
       </ReactFlow>
       {!graph.nodes.length ? (
-        <div className="dataflow-canvas-empty">Drop a source here</div>
+        <StatePanel className="definition-graph-canvas-empty" size="fill" description="Drop a source here" />
       ) : null}
     </div>
   );