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/Footer.tsx

@@ -24,7 +24,7 @@ export default function Footer() {
               Your Creative Toolkit
             </p>
             <div className="mt-4 inline-flex items-center gap-2 px-3 py-1 rounded-full bg-purple-500/10 border border-purple-500/20">
-              <span className="text-xs font-medium text-purple-400">4 Tools</span>
+              <span className="text-xs font-medium text-purple-400">5 Tools</span>
               <span className="text-gray-600">•</span>
               <span className="text-xs text-gray-500">Open Source</span>
             </div>
@@ -64,6 +64,7 @@ export default function Footer() {
               <a href="https://paint.kit.pivoine.art" className="text-gray-400 hover:text-orange-400 transition-colors text-sm">Paint</a>
               <a href="https://pastel.kit.pivoine.art" className="text-gray-400 hover:text-purple-400 transition-colors text-sm">Pastel</a>
               <a href="https://stirling.kit.pivoine.art" className="text-gray-400 hover:text-blue-400 transition-colors text-sm">Stirling</a>
+              <a href="https://units.kit.pivoine.art" className="text-gray-400 hover:text-cyan-400 transition-colors text-sm">Units</a>
             </div>
           </div>
         </motion.div>