Fix UI media - #4204

Summary

Improve Media grid scrolling performance by reducing synchronous filesystem work and removing redundant image prefetching.

Changes

• Removed UICollectionViewDataSourcePrefetching image loading.
• Replaced UIKit-driven prefetching with the existing cache-window loading mechanism.
• Avoided synchronous preview reads from prefetchItemsAt while scrolling.
• Reduced duplicate image-loading paths between collection view prefetching and the media cache window.
• Simplified preview file validation while preserving the check for zero-byte files created by the File Provider extension.
• Kept validation for all generated preview sizes: 256, 512, and 1024.
• Optimized visible-cell updates by using the original indexPath instead of scanning all visible cells.
• Prevented duplicate disk reads for the same ocId, etag, and preview size.
• Moved fallback image loading to cancellable utility-priority tasks.
• Ensured reused cells are updated only when their identifier still matches the requested media.
• Preserved the fallback preview download and placeholder metadata hydration flow.

Result

The Media grid performs less filesystem I/O during scrolling, avoids duplicate image loads, and provides smoother scrolling while keeping preview generation and cache behavior unchanged.

🤖 AI (if applicable)

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