style: reduce padding on sign-in message to match composer

Change from px-3 py-2 to px-2 py-1 to match the horizontal and vertical
padding of the logged-in message composer (px-2 py-1), ensuring
consistent height between logged-in and logged-out states.

src/components/ChatViewer.tsx

@@ -1103,7 +1103,7 @@ export function ChatViewer({
           {uploadDialog}
         </div>
       ) : (
-        <div className="border-t px-3 py-2 text-center text-sm text-muted-foreground">
+        <div className="border-t px-2 py-1 text-center text-sm text-muted-foreground">
           Sign in to send messages
         </div>
       )}