refactor(media): move image cache window management into NCImageCache

[marinofaggiana]

Summary

This PR moves the image cache window management out of NCMedia and into NCImageCache, making the cache responsible for its own lifecycle and state.

Changes

• Move cache window state (centerIndex, cache task, missing images, extension tracking) into NCImageCache.
• Introduce ImageCacheWindowItem to decouple cache management from media metadata.
• Move cache window update and preload logic into NCImageCache.
• Build cache window items once in NCMediaDataSource instead of recreating them on every scroll.
• Simplify NCMedia by delegating all cache window management to NCImageCache.
• Clear the cache window state together with the image cache on memory warnings, background transitions, and layout changes.
• Replace MainActor synchronization with an internal actor dedicated to cache window state, keeping cache management independent from the UI thread.

Benefits

• Clear separation of responsibilities.
• Less work performed during scrolling.
• Reduced object allocations by reusing precomputed cache window items.
• Better concurrency by isolating mutable cache state inside an actor instead of relying on the main thread.
• Improved maintainability and easier future evolution of the media cache.

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI