style: improve mobile responsiveness of Activity Feed

[wheval]

Summary

• Improve Activity Feed readability on mobile by rendering payments as stacked cards on small screens.
• Preserve the existing desktop table layout from the sm breakpoint and above.
• Adjust loading and empty-state spacing so the component remains consistent with the Drips Wave theme across breakpoints.

Test plan

• Open Activity Feed on a mobile viewport and confirm entries render as cards without horizontal scrolling.
• Resize to desktop and confirm the original table layout still appears and behaves correctly.
• Validate loading, empty, and error states render correctly on both mobile and desktop.

Closes #565

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