✨ feat: add pinned posts functionality

[welpo]

Summary

Add ability to pin posts to the top of the main page listing.

Posts marked with pinned = true in their front matter will appear first while maintaining their relative sort order within the section's sort_by.

Related issue

Idea from #406.

Changes

• Add pinned posts detection and sorting in list_posts.html
• Adds i18n string
• Adds a bit of CSS
• Sets Mastering tabi post as pinned

Accessibility

• Uses aria-hidden="true" to decorative SVG icon
• Pin status is conveyed through visible text in the user's language
• Maintains semantic HTML structure using list elements

Screenshots

Type of change

• Bug fix (fixes an issue without altering functionality)
• New feature (adds non-breaking functionality)
• Breaking change (alters existing functionality)
• UI/UX improvement (enhances user interface without altering functionality)
• Refactor (improves code quality without altering functionality)
• Documentation update
• Other (please describe below)

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Checklist

• I have verified the accessibility of my changes
• I have tested all possible scenarios for this change
• I have updated theme.toml with a sane default for the feature
• I have made corresponding changes to the documentation:
  • Updated config.toml comments
  • Updated theme.toml comments
  • Updated "Mastering tabi" post in English
  • (Optional) Updated "Mastering tabi" post in Spanish
  • (Optional) Updated "Mastering tabi" post in Catalan

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[welpo]

@TheAwiteb how's pinned = "مثبت"?

I used Twitter's "Pinned Tweet" phrasing as reference.