feat: remove index.tsx and update App.tsx

frontend/src/App.tsx

@@ -0,0 +1,14 @@
+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>
+  );
+}

frontend/src/index.tsx

@@ -0,0 +1 @@
+// This file can be removed since Next.js handles the entry point automatically