Fix podcast iframe: initialize music_player_html and add fallback

src/app.py

@@ -668,6 +668,9 @@ def play_next_segment():
         intro = segment.get("intro", "")
         podcast = segment.get("podcast", {})
         
+        # Initialize music_player_html for podcast
+        music_player_html = ""
+        
         if intro and getattr(tts_service, "available", True):
             audio_bytes = tts_service.text_to_speech(intro)
             if audio_bytes: