chore: make this a useful placeholder tab

apps/expo/src/app/(tabs)/downloads.tsx

@@ -0,0 +1,10 @@
+import ScreenLayout from "~/components/layout/ScreenLayout";
+import { Text } from "~/components/ui/Text";
+
+export default function DownloadsScreen() {
+  return (
+    <ScreenLayout title="Downloads">
+      <Text>Downloads tab</Text>
+    </ScreenLayout>
+  );
+}