feat: add prev/next article navigation with shell prompt style
Add top and bottom navigation between sequential articles with hacker aesthetic:
- Top nav: [visitor@danix.xyz articles]$ cd
- Bottom nav: [visitor@danix.xyz articles]$ ls ../
- Missing link shows dimmed placeholder (beginning/end)
- Only renders on articles, not static pages
- New partial: article-nav.html
- Styling: monospace prompt in accent color, hover links with transition
