feat: migrate to Next.js 14 native sitemap with optimized splitting

[FinleyGe]

Summary

• Remove next-sitemap dependency for better performance and maintainability
• Implement Next.js 14 native sitemap generation using MetadataRoute.Sitemap
• Split sitemap into logical sections for SEO best practices:
  • Main sitemap index (/sitemap.xml)
  • Base pages sitemap (/sitemap-base.xml) - 13 URLs, 5.6KB
  • FAQ pages sitemap (/sitemap-faq.xml) - 4200 URLs, 2.2MB
• Add proper URL encoding for FAQ IDs to handle special characters like &
• Include comprehensive hreflang annotations for multilingual SEO
• Implement proper XML generation with caching headers
• Update package.json to remove postbuild script
• Add native robots.txt generation

Test Plan

• Verify sitemap XML format is valid
• Test all sitemap endpoints return 200 status
• Confirm FAQ detail pages are accessible
• Validate multilingual hreflang annotations
• Check file sizes are within recommended limits (<50MB, <50K URLs)
• Ensure all FAQ URLs are included (4200 URLs across 3 languages)
• Test production build and serve

SEO Benefits

• Better Crawl Efficiency: Split sitemaps allow search engines to process content more efficiently
• Improved Performance: Smaller file sizes reduce download time
• Native Support: No third-party dependencies, directly integrated with Next.js 14
• Enhanced Multilingual Support: Complete hreflang annotations for international SEO
• Proper Standards Compliance: Follows sitemaps.org protocol and best practices

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