Restore the footer avatar's transparent background

The first WebP was rasterised through macOS Quick Look, which composites
SVGs onto an opaque white canvas. With border-radius the avatar then read
as a face floating on a white disc instead of blending into the footer —
glaring in dark mode.

Re-render the source SVG through an HTML canvas, which starts transparent,
and export WebP with its alpha channel intact (confirmed Alpha: 1). Still
tiny at 6.8 KB, and it now sits cleanly on any footer background.