feat: add width, height as props to App.tsx (#1871)

src/types.ts

@@ -79,6 +79,8 @@ export type AppState = {
   /** group being edited when you drill down to its constituent element
     (e.g. when you double-click on a group's element) */
   editingGroupId: GroupId | null;
+  width: number;
+  height: number;
 };
 
 export type PointerCoords = Readonly<{