style: align default styles with VS Code markdown preview

Update spacing, colors, and typography to match VS Code's markdown
preview rendering. Key changes: heading margins/weights/borders,
paragraph spacing, blockquote left-border, code block/inline styling,
list line-heights, table borders, link color, and base text size.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: mientjan