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).

docs/DEVICE_TESTING.md

@@ -1,7 +1,7 @@
 # Device testing checklist
 
-Simulator verification covered connect/disconnect, transcripts (text mode), media
-reconciliation/UI, tasks panel, and the hardening pass. The items below need a **real
+Simulator verification covered connect/disconnect, transcripts (text mode), tasks
+panel, and the hardening pass. The items below need a **real
 iPhone on the LAN** (`https://192.168.86.2:9178`) because mic input, WebRTC capture,
 and audio session behavior don't work in this simulator environment (CoreAudio `-4010`,
 0 input channels).
@@ -31,29 +31,18 @@ ensure the server cert pin is current (see README — re-export after any server
     side button to lock the device. It only disables the automatic idle timeout.
 - [ ] After hang-up, auto-lock returns (idle timer re-enabled).
 
-## 4. Audible media playback
-
-- [ ] The deployed server now serves cached AAC for library tracks requested with
-    `?format=m4a`; verify actual speaker/headphone output on hardware.
-- [ ] Play a library track: audio is audible, now-playing card shows art/title/status.
-- [ ] Pause / ±10 s / stop / seek all work; agent mics off while playing
-    (`media_state` published), mics back on when stopped.
-- [ ] End conversation while music was playing before → playback resumes.
-
-## 5. Audio interruption (phone call / Siri)
+## 4. Audio interruption (phone call / Siri)
 
 - [ ] Mid-conversation, trigger an interruption (incoming call or "Hey Siri").
-- [ ] Mic is muted on interruption begin; conversation/media pause cleanly.
-- [ ] On end (with `shouldResume`), mic and media resume automatically.
-- [ ] Media panel: AVPlayer pauses on interruption and resumes after.
+- [ ] On end (with `shouldResume`), mic and conversation resume automatically.
 
-## 6. Mic permission denied fallback
+## 5. Mic permission denied fallback
 
 - [ ] Deny microphone permission in iOS Settings for the app, relaunch.
 - [ ] Start → session reaches **ready** despite mic failure (non-fatal); orange error
     banner shows the mic error; text mode remains fully usable.
 
-## 7. General device sanity
+## 6. General device sanity
 
 - [ ] Connect / hang-up / reconnect matrix (3×) — no crashes, state resets cleanly.
 - [ ] Change server URL in Settings while idle → session recreates without crash.