README demos: frame each animation as a rounded tile

A margin of the page backdrop around the figure, corners rounded to
transparency and a hairline border, so the tiles sit on either GitHub theme
instead of ending in a hard crop.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WaquS6UzvpZEbiGp6w3LfV

Author: uppertoe