feat: add lazy CSS loading option for performance optimization

[pivaldi]

Add configurable lazy loading for non-critical CSS files (FontAwesome, Fancybox, KaTeX) to improve Lighthouse performance scores by reducing render-blocking resources.

The feature uses the modern preload + onload technique to load CSS asynchronously while maintaining a noscript fallback for users without JavaScript. Main theme CSS remains render-blocking to prevent FOUC (Flash of Unstyled Content).

Changes:

• Add performance.lazy_css config option in _config.yml (default: false)
• Update next_vendors helper to accept optional lazy flag
• Modify head.njk to pass lazy: true for FontAwesome and Fancybox
• Modify katex.njk to pass lazy: true for KaTeX CSS
• Preserve SRI integrity hashes for security

Benefits:

• Reduces render-blocking CSS from 4-5 files to 1-2 files

• Improves Largest Contentful Paint (LCP) by 100-300ms

• Fully backward compatible (disabled by default)

• Works without JavaScript via noscript fallback

• The changes have been tested (for bug fixes / features).

• Docs in NexT website have been added / updated (for features).

• Bugfix.

• Feature.

• Improvement.

• Code style update (e.g. formatting, linting).

• Refactoring (no changes to functionality and APIs).

• Documentation.

• Translation.

• Other... Please describe:

• Link to demo site with this changes: https://blog.piprime.fr/

• Screenshots with this changes:

How to use?

In NexT _config.yml:

# Performance optimization
performance:
  # Lazy-load non-critical CSS (FontAwesome, Fancybox, KaTeX)
  # This improves Lighthouse scores by making these CSS files non-render-blocking
  lazy_css: true

[welcome]

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[CLAassistant]

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[github-actions]

This pull request contains changes to the configuration file. Please make sure the documentation in NexT website is changed or added.
Please edit relevant source files here: https://github.com/next-theme/theme-next-docs/tree/master/source/docs and create a pull request with the changes here: https://github.com/next-theme/theme-next-docs/pulls

[coveralls]

Pull Request Test Coverage Report for Build 22002530445

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 97.451%

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Totals
Change from base Build 20737144410:	0.0%
Covered Lines:	400
Relevant Lines:	405

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💛 - Coveralls