Add clickable tag filtering

Make tags on the homepage clickable so users can view all content filtered by a single tag. Create a /tags/[tag] page that displays entries from all categories mixed together.

Completed

• ✅ Update homepage emoji (🧠 → 🦞)
• ✅ Convert tag span to Link component
• ✅ Add getContentByTag() utility
• ✅ Create /tags/[tag] detail page
• ✅ Generate static pages for all tags at build time
• ✅ Display entry metadata (category, date, tags)