Fix background playback setting behavior across platforms

Hide the toggle on macOS, where apps are never suspended and playback
always continues regardless of the setting. Pause playback on iOS when
backgrounding with the setting disabled — the audio background mode
previously kept mpv playing, making the off state a no-op.

Move the continue-playing policy into PlayerService and make the
backend's background render pause unconditional (PiP-gated only):
it is a GPU optimization that should not depend on the setting.
Also ungate the player sheet visibility handlers, which manage
foreground mini player rendering unrelated to background playback.

Yattee/Views/Settings/PlaybackSettingsView.swift

@@ -270,10 +270,14 @@ private struct BehaviorSection: View {
                 }
             }
 
+            #if !os(macOS)
+            // macOS never suspends apps, so playback always continues in the
+            // background — the toggle would have no effect there
             Toggle(
                 String(localized: "settings.playback.backgroundPlayback"),
                 isOn: $settings.backgroundPlaybackEnabled
             )
+            #endif
 
             #if os(tvOS)
             Toggle(