Add duplicate import checker and non-duplicate generator

[akriaueno]

Summary

• Avoid duplicate kit wrapper imports in generate-kit-docs.js
• Detect duplicate imports inside Sandpack code in scripts/check-docs-mdx.mjs

Changes

• Add import statement extraction and named import checks; only inject colors/Panel when needed
• Parse files={{ ... }} blocks and use TypeScript to flag duplicate imports as errors
• Regenerate docs/default-kit/* and docs/horizon-kit/* with generate-kit-docs.js

Ref: #244