feat: media player with reconciliation and pinned media proxy (Phase 5)

- MediaAPI client for all 10 media endpoints + Codable models
- PinnedMediaResourceLoader: AVAssetResourceLoaderDelegate proxying
  https media through the pinned-TLS session (AVPlayer ignores registered
  URLProtocol subclasses and can't trust the self-signed cert)
- MediaPlayerController: AVPlayer + 1.5s poll reconciliation ported from
  web app.js (track swap, late audio_url, desired state, seek_hint,
  reporting, enter-media-scope carry-over, post-talk resume)
- MediaPanelView UI + music.note toolbar entry
- VoiceSession wiring (enterMediaScope/endConversation)

Playback of the library's webm tracks is blocked by an iOS platform
limitation (AVPlayer cannot decode webm); TLS/Range proxy, reconciliation,
polling, reporting and UI are all verified working in the simulator.

Sources/Features/Conversation/ConversationView.swift

@@ -172,7 +172,15 @@ struct ConversationView: View {
         .padding()
     }
 
+    @ToolbarContentBuilder
     private var toolbarContent: some ToolbarContent {
+        ToolbarItem(placement: .topBarTrailing) {
+            NavigationLink {
+                MediaPanelView(playerController: session.mediaController, settings: settings)
+            } label: {
+                Image(systemName: "music.note")
+            }
+        }
         ToolbarItem(placement: .topBarTrailing) {
             NavigationLink {
                 SettingsView(settings: settings, session: session)