Fix properties panel reactivity: spread node objects in computed

Vue's computed caching compares array contents by reference. Since
graph.getNode() returns the same mutable object, the computed saw
no change even when properties were mutated. Spreading creates new
objects so Vue detects the difference.

src/stores/editor.ts

@@ -103,7 +103,7 @@ export function createEditorStore() {
     const nodes: SceneNode[] = []
     for (const id of state.selectedIds) {
       const n = graph.getNode(id)
-      if (n) nodes.push(n)
+      if (n) nodes.push({ ...n })
     }
     return nodes
   })