Layout is not responsive on small screens

src/index.css has no responsive rule; add a simple @media rule / flex-wrap so it looks good on mobile.

Start in src/index.css and inspect the existing layout rules for the elements that overflow or fail to wrap on narrow screens. Run the app and check it at a small mobile viewport while adjusting the responsive rule or flex wrapping. Done means the layout remains usable and visually organized on small screens without horizontal overflow.

Written by the indexing model from the issue text.