docs: round README screenshot corners

[16francej]

Spec diff: none.

Summary

• Mask the README hero screenshot's four corner wedges to transparency.
• Preserve the original 3024×1394 dimensions and every opaque UI pixel.

Why

The PNG had an alpha channel, but every pixel was fully opaque. The background pixels outside the application's corners therefore made the screenshot canvas appear square.

Impact

Documentation asset only; runtime behavior is unchanged.

After

Validation

• Confirmed the output remains 3024×1394.
• Confirmed all fully opaque RGB pixels are unchanged.
• Confirmed 448 fully transparent and 196 antialiased edge pixels across the four corners.
• git diff --check