Slideshow audio respects what you're already listening to

I often want the slideshow running while music or a podcast is already playing. The app now treats that audio as something to respect: muted media leaves it alone, and audible motion interrupts it only when there is actually something worth hearing.

Changelog

• Muted slideshow videos now preserve music, podcasts, and other audio already playing on Apple TV.
• When slideshow audio is enabled, music can continue quietly underneath while Apple TV-recognized spoken audio is paused during video or Live Photo motion.
• Background audio is restored promptly when slideshow motion pauses, finishes, or is skipped.