fix: resolve build errors and warnings

frontend/src/app/layout.tsx

@@ -1,3 +1,5 @@
+"use client";
+
 import React from "react";
 import "./globals.css";
 
@@ -17,7 +19,6 @@ export default function RootLayout({
         <div className="container">
           <header className="py-4 border-b">
             <h1 className="text-2xl font-bold">ポッドキャスト管理画面</h1>
-          </h1>
           </header>
           <main className="py-6">{children}</main>
           <footer className="py-4 border-t text-center text-gray-500">

frontend/src/components/EpisodePlayer.tsx

@@ -1,3 +1,5 @@
+"use client";
+
 import React, { useState } from "react";
 
 interface Episode {

frontend/src/components/FeedList.tsx

@@ -1,3 +1,5 @@
+"use client";
+
 import React, { useEffect, useState } from "react";
 
 interface FeedItem {