Fix thumbnail aspect ratio in video grid cells

Thumbnails were being stretched vertically due to incorrect aspect ratio handling. Fixed by:
- Using .scaledToFill() on thumbnails to fill the container width
- Constraining container to 16:9 aspect ratio with .fit mode
- Adding matching aspect ratio to placeholder to prevent layout shift during loading

This ensures thumbnails maintain proper proportions while filling the full cell width.

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 {
@@ -76,6 +76,8 @@ struct ThumbnailView: View {
     }
 
     var placeholder: some View {
-        Rectangle().fill(Color("PlaceholderColor"))
+        Rectangle()
+            .fill(Color("PlaceholderColor"))
+            .aspectRatio(Constants.aspectRatio16x9, contentMode: .fit)
     }
 }

Shared/Videos/VideoCell.swift

@@ -429,16 +429,14 @@ struct VideoCell: View {
             }
             .lineLimit(1)
         }
+        .aspectRatio(Constants.aspectRatio16x9, contentMode: .fit)
     }
 
     private var thumbnailImage: some View {
-        Group {
         VideoCellThumbnail(video: video)
-
             #if os(tvOS)
             .frame(minHeight: 320)
             #endif
-        }
             .mask(RoundedRectangle(cornerRadius: thumbnailRoundingCornerRadius))
     }
 
@@ -477,11 +475,13 @@ struct VideoCellThumbnail: View {
     private var thumbnails: ThumbnailsModel { .shared }
 
     var body: some View {
-        let (url, quality) = thumbnails.best(video)
+        let (url, _) = thumbnails.best(video)
-        let aspectRatio = (quality == .default || quality == .high) ? Constants.aspectRatio4x3 : Constants.aspectRatio16x9
 
         ThumbnailView(url: url)
-            .aspectRatio(aspectRatio, contentMode: .fill)
+            .scaledToFill()
+            .frame(maxWidth: .infinity)
+            .aspectRatio(Constants.aspectRatio16x9, contentMode: .fit)
+            .clipped()
     }
 }