Replaces stub implementation with full related content functionality:
- Uses Hugo's .Site.RegularPages.Related API for smart tagging
- Caps at 5 related articles to keep sidebar compact
- Shows 2-line compact card per entry (title + date, excerpt snippet)
- Left border colored by article type (var(--type-<type>))
- Includes empty state message when no related articles found
- Multilingual support via existing i18n keys
- Theme-aware styling via CSS custom properties
The widget shows:
- Line 1: Title (linked) + ยท Jan 2006 date inline
- Line 2: First ~10 words of excerpt, small font, muted
Previously was a stub with empty placeholder comment.
