Keep managed iOS audio active in background

clients/apple/dotnet/VoiceCat.iOS/AppModel.cs

@@ -32,6 +32,7 @@ internal sealed class AppModel
     private uint restoreChannel;
     private bool restoreMuted;
     private bool restoreDeafened;
+    private bool backgrounded;
 
     internal event Action? Changed;
     internal event Action<ServerIdentityChallenge>? IdentityRequested;
@@ -44,6 +45,7 @@ internal sealed class AppModel
     internal bool IsConnecting { get; private set; }
     internal bool VoiceJoined => microphoneStream != 0;
     internal bool ScreenSharing => broadcast?.IsActive == true;
+    internal bool IsBackgrounded => backgrounded;
     internal float MicrophoneLevel { get; private set; }
     internal string Status { get; private set; } = "Not connected";
     internal uint CurrentChannelId { get; private set; }
@@ -55,6 +57,27 @@ internal sealed class AppModel
     internal void Load() { profiles.Clear(); profiles.AddRange(storage.LoadProfiles()); settings.Load(); IosAudioRouter.Shared.Load(); Notify(); }
     internal void Save() { storage.SaveProfiles(profiles); settings.Save(); }
 
+    internal void DidEnterBackground()
+    {
+        backgrounded = true; Save();
+        // Do not stop or rebuild AVAudioEngine here. With the `audio` background mode and an
+        // active PlayAndRecord session, capture, playback, media UDP, and screen-ring draining
+        // remain live while the scene is backgrounded or the device is locked.
+    }
+
+    internal void WillEnterForeground()
+    {
+        backgrounded = false;
+        IosAudioRouter.Shared.Recover("foreground");
+    }
+
+    internal void DidBecomeActive()
+    {
+        backgrounded = false;
+        IosAudioRouter.Shared.EnsureAudio("active scene");
+        Notify();
+    }
+
     internal void UpsertProfile(ServerProfile profile, string? password)
     {
         int index = profiles.FindIndex(item => item.Id == profile.Id);
@@ -186,6 +209,17 @@ internal sealed class AppModel
         microphoneStream = stream.StreamId; IosAudioEngine.Shared.StartMicrophone(stream.StreamId, channels); feedback.Play(SoundEvent.VoiceOn); Notify();
     }
 
+    internal void ToggleScreenAudio()
+    {
+        if (!OperatingSystem.IsIOSVersionAtLeast(27)) throw new PlatformNotSupportedException("Use the ReplayKit broadcast picker on this iOS version.");
+        if (ScreenSharing) { IosScreenCapture.Stop(); broadcast?.RequestStop(); }
+        else
+        {
+            if (!IsConnected || CurrentChannelId == 0) throw new InvalidOperationException("Connect and join a channel before sharing screen audio.");
+            IosScreenCapture.Present();
+        }
+    }
+
     internal void SetSelfAudio(bool muted, bool deafened) { client?.SetSelfAudioState(muted, deafened); Notify(); }
     internal void SetPushToTalk(bool active)
     {