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/LegacyDataImportView.swift

@@ -38,6 +38,7 @@ struct LegacyDataImportView: View {
                     systemImage: "doc.questionmark",
                     description: Text(String(localized: "migration.noDataFoundDescription"))
                 )
+                .frame(maxWidth: .infinity, maxHeight: .infinity)
             } else {
                 importContent
             }