Fix API availability issues for macOS 11.0 and tvOS 15.0

This commit resolves multiple build errors caused by using APIs that
require newer OS versions than the deployment targets (macOS 11.0 and
tvOS 15.0).

macOS fixes:
- Add missing init(frame:) initializer to PlayerLayerView
- Add availability checks for textSelection modifier (macOS 12.0+)
- Add availability checks for AttributedString (macOS 12.0+)
- Add availability checks for listStyle.inset(alternatesRowBackgrounds:) (macOS 12.0+)
- Add availability checks for focusScope modifier (macOS 12.0+)
- Correct listRowSeparator availability from macOS 12.0 to 13.0

tvOS fixes:
- Use older onChange(of:) signature compatible with tvOS 15.0
- Add availability check for Menu with primaryAction (tvOS 17.0+)

All changes include appropriate fallbacks for older OS versions to
maintain backward compatibility.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Shared/Channels/ChannelVideosView.swift

@@ -181,15 +181,19 @@ struct ChannelVideosView: View {
         .navigationTitle(navigationTitle)
         #endif
 
-        return Group {
-            content
-            #if os(tvOS)
-            .background(Color.background(scheme: colorScheme))
-            #endif
-            #if !os(iOS)
-            .focusScope(focusNamespace)
-            #endif
-        }
+        #if os(tvOS)
+            return content
+                .background(Color.background(scheme: colorScheme))
+                .focusScope(focusNamespace)
+        #elseif os(macOS)
+            if #available(macOS 12.0, *) {
+                return content.focusScope(focusNamespace)
+            } else {
+                return content
+            }
+        #else
+            return content
+        #endif
     }
 
     var verticalCellsEdgesIgnoringSafeArea: Edge.Set {