[Kickoff] Fix default avatar offset

Nowadays we place avatars inside a circle with an always visible border.

The default avatar has not been adjusted for this and has both its own dimensions and positioning.

This patch has it fill the avatar circle and shifts it upwards a bit because otherwise the avatar overflows the circle when scaling is increased.

Anchors.bottomMargin is used instead of anchors.margins because the latter results in a blurry SVG, while the former doesn't and looks just fine.

BUG: 414687