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

- 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

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Co-Authored-By: Claude <[email protected]>
Co-Authored-By: Happy <[email protected]>

Author: 3 people