refactor: use relative paths for API calls

frontend/src/components/EpisodePlayer.tsx

@@ -21,9 +21,7 @@ export default function EpisodePlayer() {
 
   const fetchEpisodes = async () => {
     try {
-      const response = await fetch(
-        `${import.meta.env["PODCAST_BASE_URL"]}/api/episodes`,
-      );
+      const response = await fetch("/api/episodes");
       if (!response.ok) {
         throw new Error("エピソードの取得に失敗しました");
       }

frontend/src/components/FeedList.tsx

@@ -19,9 +19,7 @@ export default function FeedList() {
 
   const fetchFeeds = async () => {
     try {
-      const response = await fetch(
-        `${import.meta.env["PODCAST_BASE_URL"]}/api/feeds`,
-      );
+      const response = await fetch("/api/feeds");
       if (!response.ok) {
         throw new Error("フィードの取得に失敗しました");
       }