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/MediaBrowser/MediaBrowserView.swift

@@ -71,12 +71,14 @@ struct MediaBrowserView: View {
                     }
                     .buttonStyle(.borderedProminent)
                 }
+                .frame(maxWidth: .infinity, maxHeight: .infinity)
             } else if files.isEmpty {
                 ContentUnavailableView {
                     Label(String(localized: "mediaBrowser.emptyFolder"), systemImage: "folder")
                 } description: {
                     Text(String(localized: "mediaBrowser.emptyFolder.description"))
                 }
+                .frame(maxWidth: .infinity, maxHeight: .infinity)
             } else {
                 fileList
             }

Yattee/Views/MediaBrowser/MediaSourcesView.swift

@@ -49,6 +49,7 @@ struct MediaSourcesView: View {
                     }
                     .buttonStyle(.borderedProminent)
                 }
+                .frame(maxWidth: .infinity, maxHeight: .infinity)
             } else {
                 sourcesList
             }