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/ThumbnailView.swift

@@ -44,13 +44,13 @@ struct ThumbnailView: View {
 
     var webImage: some View {
         WebImage(url: url)
+            .resizable()
             .onFailure { _ in
                 if let url {
                     thumbnails.insertUnloadable(url)
                 }
             }
             .placeholder { placeholder }
-            .resizable()
     }
 
     @ViewBuilder var asyncImageIfAvailable: some View {
@@ -78,6 +78,6 @@ struct ThumbnailView: View {
     var placeholder: some View {
         Rectangle()
             .fill(Color("PlaceholderColor"))
-            .aspectRatio(Constants.aspectRatio16x9, contentMode: .fit)
+            .aspectRatio(Constants.aspectRatio16x9, contentMode: .fill)
     }
 }