feat: add Units converter toolkit

Added Units as the 5th tool in the kit:

🔄 Units Converter Features:
- 187 units across 23 measurement categories
- Real-time bidirectional conversion
- Smart fuzzy search for quick unit selection
- Conversion history with localStorage
- Categories: length, mass, temperature, time, angle, area, volume, etc.

🎨 Visual:
- Cyan-purple gradient
- Bidirectional arrows icon (conversion symbol)
- Badges: Real-time, Free

📊 Updates:
- Stats section: 4 → 5 Tools
- Footer: Added Units link with cyan hover
- README: Added tool description
- Grid layout: Now displays 5 tools responsively

🔗 URL: https://units.kit.pivoine.art

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

components/Stats.tsx

@@ -4,7 +4,7 @@ import { motion } from 'framer-motion';
 
 const stats = [
   {
-    number: '4',
+    number: '5',
     label: 'Tools',
     icon: (
       <svg className="w-6 h-6" fill="none" stroke="currentColor" viewBox="0 0 24 24">