Remove embedded music player

The music feature is retired: delete the Media tab, MediaAPI,
MediaPlayerController, PinnedMediaResourceLoader, MediaPanelView, the
media_state control publish, media scope handling in VoiceSession, the
volume setting, and all media tests. App is now Talk + Activity tabs.

Backend media API in hope-voice-api is untouched (still serves the web UI).

Sources/LiveKit/ControlChannel.swift

@@ -2,7 +2,7 @@ import Foundation
 import LiveKit
 
 /// Outbound helpers for agent control topics and inbound parsing of
-/// voice-control / conversation-control / media-control data messages.
+/// voice-control / conversation-control data messages.
 @MainActor
 final class ControlChannel: NSObject, RoomDelegate {
 
@@ -109,11 +109,6 @@ final class ControlChannel: NSObject, RoomDelegate {
         try await publish(["type": "set_mode", "mode": mode], topic: "conversation-control", room: room)
     }
 
-    func sendMediaState(_ state: String, trackID: String?, room: Room) async throws {
-        var object: [String: Any] = ["type": "media_state", "state": state]
-        if let trackID { object["track_id"] = trackID }
-        try await publish(object, topic: "media-control", room: room)
-    }
 }
 
 // MARK: - Minimal JSON value decoder for flexible agent payloads