feat(blog): Add DigestEntry component and migrate digest series to MDX

[planeyang]

Summary

• Create DigestEntry component for structured article entries with title, href, category, and MDX content support
• Migrate all 22 digest files (001-022) from Hugo shortcode format to DigestEntry component format
• Add Image component alias to MDXComponents for markdown image support
• Create reading-digest-zh.mdx series page
• Extract source URLs and categories from original Hugo files

Changes

New Component: DigestEntry

A card-style component for curated article entries with:

• Title as a clickable link (with external link indicator for external URLs)
• Category badge
• MDX content support for detailed commentary

Digest File Migration

All 22 digest files converted from:

{{< digest-item "source"="URL" "description"="CATEGORY" >}}
### Article Title
{{< /digest-item >}}
Content here...

To:

<DigestEntry title="Article Title" href="URL" category="CATEGORY">

Content here...

</DigestEntry>

MDX Components

Added Image component alias that wraps Figure/FigureImage for compatibility with common MDX image patterns.

Test plan

• Verify digest pages render correctly at /periodics/digest-XXX
• Check DigestEntry component displays title, category, and content
• Verify external links open in new tab with indicator
• Confirm images render properly (e.g., digest-014 has an image)

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