Adjust tvOS video cell dimensions for better layout

Reduced video cell and grid item sizes on tvOS to improve layout spacing
and visual consistency. Changed grid item size from 600 to 560 pixels,
and adjusted video cell frame dimensions accordingly.

Shared/Videos/VerticalCells.swift

@@ -84,7 +84,7 @@ struct VerticalCells<Header: View>: View {
         #if os(iOS)
             return verticalSizeClass == .regular ? 320 : 800
         #elseif os(tvOS)
-            return 600
+            return 560
         #else
             return 320
         #endif
@@ -92,7 +92,7 @@ struct VerticalCells<Header: View>: View {
 
     var adaptiveGridItemMaximumSize: Double {
         #if os(tvOS)
-            return 600
+            return 560
         #else
             return .infinity
         #endif