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/App/SettingsStore.swift

@@ -14,16 +14,12 @@ final class SettingsStore: ObservableObject {
     @Published var textMode: Bool {
         didSet { AppPreferences.store.set(textMode, forKey: Keys.textMode) }
     }
-    @Published var volume: Double {
-        didSet { AppPreferences.store.set(volume, forKey: Keys.volume) }
-    }
 
     private enum Keys {
         static let serverURL = "serverURL"
         static let voiceID = "voiceID"
         static let modelID = "modelID"
         static let textMode = "textMode"
-        static let volume = "volume"
     }
 
     static let defaultServerURL = "https://192.168.86.2:9178"
@@ -47,7 +43,6 @@ final class SettingsStore: ObservableObject {
         voiceID = Voices.all.contains { $0.id == storedVoice } ? (storedVoice ?? "") : Voices.defaultID
         modelID = d.string(forKey: Keys.modelID)
         textMode = d.bool(forKey: Keys.textMode)
-        volume = d.object(forKey: Keys.volume) as? Double ?? 0.8
     }
 
     var trimmedServerURL: String {