Add scene graph viewer and persist migration (#115)

Render an interactive Scene Graph in the Settings panel using @visual-json/react. Introduces SceneNode/SceneGraphNode types and buildSceneGraphValue to build a tree (handles missing nodes and cycles), and adds handlers to block drag/drop/context/delete mutations inside the viewer. Adds @visual-json/react to apps/editor package.json and updates the dev script to forbid .env.local and source the root .env instead. Also adds a persistence migration in use-scene to preserve existing nodes and rootNodeIds when the persist version changes.

packages/core/src/store/use-scene.ts

@@ -155,6 +155,9 @@ const useScene: UseSceneStore = create<SceneState>()(
     {
       name: 'editor-storage',
       version: 1,
+      // Keep existing local scenes when the persist version changes.
+      migrate: (persistedState) =>
+        persistedState as Pick<SceneState, 'nodes' | 'rootNodeIds'>,
       partialize: (state) => ({
         nodes: Object.fromEntries(
           Object.entries(state.nodes).filter(([_, node]) => {