展示用モードのフラグを取得する関数を追加

src/lib/exhibitionMode.ts

@@ -0,0 +1,7 @@
+export const isExhibitionMode = (): boolean => {
+  const params = new URLSearchParams(window.location.search)
+  const exhibition = params.get("exhibition")
+
+  // `exhibition`が定義されていればtrueを返す
+  return exhibition !== null
+}