feat: SEO optimization, favicons, and dark mode download button fix
- Add comprehensive metadata: Open Graph, Twitter Card, keywords, robots
- Add per-page metadata with i18n alternates for homepage (en/zh)
- Add robots.ts, sitemap.ts (dynamic from docs), manifest.ts
- Add favicon (icon.png 32px), apple-icon.png (180px), PWA icons (192/512px)
- Add og-image.png (1200x630) for social sharing
- Update site.config.ts with canonical URL and improved description
- Fix DownloadButton dark mode: white-on-white text → dark:bg-white dark:text-black
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
