perf: lazy-load images in markdown preview

[shrutig-blip]

Summary

Adds a custom image renderer to the Markdown preview to improve loading performance.

Changes

• Add a custom img renderer in MarkdownPreview
• Set loading="lazy" for off-screen images
• Set decoding="async" for asynchronous image decoding
• Preserve existing image props (src, title, etc.) by spreading the props
• Default missing alt text to "" for accessibility
• Keep using the native <img> element since Markdown images can reference arbitrary user-supplied URLs, making next/image unsuitable

Testing

• npm run lint
• npm run build

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[shrutig-blip]

Hi @Rinava! I've addressed #43 by adding a custom image renderer that lazy-loads Markdown images while preserving the existing props and providing an empty alt fallback. I also kept the native with a scoped lint suppression since the image URLs are arbitrary user-supplied Markdown URLs. I'd appreciate any feedback. Thanks!