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/Views/Settings/PlaybackSettingsView.swift

@@ -271,12 +271,10 @@ private struct BehaviorSection: View {
                 }
             }
 
-            #if os(iOS) || os(macOS)
             Toggle(
                 String(localized: "settings.playback.backgroundPlayback"),
                 isOn: $settings.backgroundPlaybackEnabled
             )
-            #endif
 
             #if os(tvOS)
             Toggle(