fix the ugliness of bare messages

- replace "loading..." messages with LoadingPlaceholder
- Use EmptyView where appropriate

src/screens/manga/SourceMangas.tsx

@@ -93,6 +93,7 @@ export default function SourceMangas(props: { popular: boolean }) {
             setLastPageNum={setLastPageNum}
             message={message}
             messageExtra={messageExtra}
+            isLoading={!fetched}
         />
     );
 }