Skill page: match iframe background to compare stage in dark mode

Embed mode used background: transparent on example.html, which let the
iframe fall back to --bg-page (#070707 dark / #fdfdfd light) instead
of the skill compare stage surface (#181818 dark / #ffffff light).
That read as a mismatched rectangle inside the stage card.

- example.html: set html[data-embed] + body embed backgrounds to
  #ffffff / #181818 to match skill.html .skill-compare-stage
- skill.html: iframe background var(--card-bg) for pre-paint parity
- Bump iframe cache-buster to v=20260520-5

Co-authored-by: Cursor <cursoragent@cursor.com>