Fix ContentUnavailableView centering on Apple TV

On tvOS, ContentUnavailableView inside a Group doesn't expand to fill
available space — it sizes to content and aligns top-leading. Add
.frame(maxWidth: .infinity, maxHeight: .infinity) to all instances
so they center correctly in their parent containers.

Yattee/Views/Settings/SourcesListView.swift

@@ -90,6 +90,7 @@ struct SourcesListView: View {
             }
             .buttonStyle(.borderedProminent)
         }
+        .frame(maxWidth: .infinity, maxHeight: .infinity)
         .accessibilityIdentifier("sources.view")
     }