fix(render): honor system mode for direct rich surfaces

[benvinegar]

Summary

• opt no-mode sandboxed surface docs into color-scheme: light dark so the browser resolves the user system mode
• let direct diff renders use @pierre/diffs system theme mode instead of defaulting to light
• make no-mode mermaid renders pick light/dark in-frame via matchMedia before rendering
• add regression coverage and a patch changeset

Validation

• npm test
• npm run typecheck
• npm run lint
• npm run format:check

This PR description was generated by Pi using OpenAI GPT-5