Use color.clear so placeholder images are not visible when selected

apple/OmnivoreKit/Sources/Views/FeedItem/GridCard.swift

@@ -77,7 +77,7 @@ public struct GridCard: View {
           CachedAsyncImage(url: imageURL) { phase in
             switch phase {
             case .empty:
-              Color.systemBackground
+              Color.clear
                 .frame(maxWidth: .infinity, maxHeight: geo.size.height)
             case let .success(image):
               image.resizable()
@@ -93,7 +93,7 @@ public struct GridCard: View {
               // we need to add this currently unused fallback
               // to handle any new cases that might be added
               // in the future:
-              Color.systemBackground
+              Color.clear
                 .frame(maxWidth: .infinity, maxHeight: geo.size.height)
             }
           }

apple/OmnivoreKit/Sources/Views/FeedItem/LibraryFeatureCard.swift

@@ -37,7 +37,7 @@ public struct LibraryFeatureCard: View {
         CachedAsyncImage(url: imageURL) { phase in
           switch phase {
           case .empty:
-            Color.systemBackground
+            Color.clear
               .frame(width: 146, height: 90)
           case let .success(image):
             image.resizable()

apple/OmnivoreKit/Sources/Views/FeedItem/LibraryItemCard.swift

@@ -312,7 +312,7 @@ public struct LibraryItemCard: View {
               .cornerRadius(5)
               .padding(.top, 2)
           } else {
-            Color.systemBackground
+            Color.clear
               .frame(width: 50, height: 75)
               .cornerRadius(5)
               .padding(.top, 2)