Expose Background Playback toggle on tvOS, default off

Surfaces the existing iOS/macOS Background Playback setting in the tvOS
Playback settings, defaulting to off so audio stops when the user leaves
the app via the TV button. Pauses playback on .background/.inactive when
the toggle is off, regardless of audio route — the user's setting wins
over AirPlay/HomePod handoff. Also auto-shows the tvOS player controls
when returning to foreground so the paused state is immediately visible
and actionable.

Yattee/Core/Settings/SettingsManager+Playback.swift

@@ -40,7 +40,7 @@ extension SettingsManager {
     var backgroundPlaybackEnabled: Bool {
         get {
             if let cached = _backgroundPlaybackEnabled { return cached }
-            return bool(for: .backgroundPlayback, default: true)
+            return bool(for: .backgroundPlayback, default: backgroundPlaybackDefault)
         }
         set {
             _backgroundPlaybackEnabled = newValue
@@ -48,6 +48,14 @@ extension SettingsManager {
         }
     }
 
+    private var backgroundPlaybackDefault: Bool {
+        #if os(tvOS)
+        return false
+        #else
+        return true
+        #endif
+    }
+
     /// tvOS only: when enabled, the Siri remote Menu button closes the video
     /// (clears queue, stops playback) instead of only collapsing the player.
     /// When enabled, the explicit top-bar close button is hidden.