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/ToolsGrid.tsx

@@ -56,6 +56,18 @@ const tools = [
       </svg>
     ),
   },
+  {
+    title: 'Units',
+    description: 'Smart unit converter with 187 units across 23 categories. Real-time bidirectional conversion with fuzzy search and conversion history.',
+    url: 'https://units.kit.pivoine.art',
+    gradient: 'gradient-cyan-purple',
+    badges: ['Real-time', 'Free'],
+    icon: (
+      <svg className="w-12 h-12 text-white" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+        <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M7 16V4m0 0L3 8m4-4l4 4m6 0v12m0 0l4-4m-4 4l-4-4" />
+      </svg>
+    ),
+  },
 ];
 
 export default function ToolsGrid() {