Fix audio session interrupting other apps on launch

Previously, the audio session was initialized immediately when the app launched, causing audio from other apps (like Music) to stop even when no video was playing in Yattee.

Changes:
- Remove audio session initialization from AppDelegate launch
- Remove audio session setup from MPVClient initialization
- Update setAudioSessionActive() to configure audio session category before activation

The audio session is now lazily initialized only when playback actually starts:
- For MPV backend: triggered by FILE_LOADED, PLAYBACK_RESTART, AUDIO_RECONFIG events
- For AVPlayer backend: triggered when play() is called

This allows music from other apps to continue playing until a video is actually played in Yattee.

iOS/AppDelegate.swift

@@ -22,13 +22,6 @@ final class AppDelegate: UIResponder, UIApplicationDelegate {
             OrientationTracker.shared.startDeviceOrientationTracking()
             OrientationModel.shared.startOrientationUpdates()
 
-            do {
-                try AVAudioSession.sharedInstance().setCategory(.playback, mode: .moviePlayback)
-                try AVAudioSession.sharedInstance().setActive(true)
-            } catch {
-                logger.error("Failed to set audio session category: \(error)")
-            }
-
             UIApplication.shared.beginReceivingRemoteControlEvents()
         #endif