feat: setup Next.js frontend structure

frontend/src/app/page.tsx

@@ -0,0 +1,18 @@
+import React from "react";
+import FeedList from "../components/FeedList";
+import EpisodePlayer from "../components/EpisodePlayer";
+
+export default function Home() {
+  return (
+    <div className="space-y-8">
+      <section>
+        <h2 className="text-xl font-semibold mb-4">フィード一覧</h2>
+        <FeedList />
+      </section>
+      <section>
+        <h2 className="text-xl font-semibold mb-4">エピソードプレイヤー</h2>
+        <EpisodePlayer />
+      </section>
+    </div>
+  );
+}