Fix thumbnail aspect ratio to prevent stretching and layout jumps

Fixed issues with thumbnails being stretched vertically and layout jumping during image loading:
- Simplified VideoCellThumbnail to always use 16:9 aspect ratio with .fill mode
- Added matching 16:9 aspect ratio to placeholder with .fill mode to prevent layout shifts
- Removed quality-based aspect ratio selection (4:3 vs 16:9) in favor of consistent 16:9
- Ensures thumbnails maintain proper proportions on both iOS and macOS

This provides consistent sizing across platforms and eliminates the jump when images finish loading.

Shared/Videos/VideoCell.swift

@@ -429,7 +429,6 @@ struct VideoCell: View {
             }
             .lineLimit(1)
         }
-        .aspectRatio(Constants.aspectRatio16x9, contentMode: .fit)
     }
 
     private var thumbnailImage: some View {
@@ -478,10 +477,7 @@ struct VideoCellThumbnail: View {
         let (url, _) = thumbnails.best(video)
 
         ThumbnailView(url: url)
-            .scaledToFill()
-            .frame(maxWidth: .infinity)
-            .aspectRatio(Constants.aspectRatio16x9, contentMode: .fit)
-            .clipped()
+            .aspectRatio(Constants.aspectRatio16x9, contentMode: .fill)
     }
 }