chore: remove frontend files

frontend/src/App.tsx

@@ -1,14 +0,0 @@
-import React from "react";
-import FeedList from "../components/FeedList";
-import EpisodeList from "../components/EpisodeList";
-
-export default function App() {
-  return (
-    <div style={{ padding: "20px", fontFamily: "sans-serif" }}>
-      <h1>ポッドキャスト自動生成サービス 管理画面</h1>
-      <FeedList />
-      <hr style={{ margin: "20px 0" }} />
-      <EpisodeList />
-    </div>
-  );
-}