Smoother slideshow preloading

Watching a slideshow should not feel like waiting for iCloud. The app now works ahead, preparing nearby media while keeping a hard limit on how much it holds, so the rotation can stay smooth without slowly consuming the Apple TV.

Changelog

• Slideshow playback now prepares nearby photos, videos, and Live Photos ahead of time to reduce visible loading interruptions.
• Recently viewed media stays responsive when navigating backward, while a fixed memory limit prevents the cache from growing during long sessions.
• Playback can continue with another prepared item when the next item is temporarily unavailable.