feat: add Stirling PDF toolkit

Added Stirling PDF as the 4th tool in the kit:

📄 Stirling PDF Features:
- 50+ PDF operations (merge, split, convert)
- OCR processing for scanned documents
- Digital signatures and watermarks
- Privacy-first: locally hosted, no cloud dependency
- Open source with 69.6k+ GitHub stars

🎨 Visual:
- Purple-blue gradient matching brand
- PDF document icon with lines
- Badges: Privacy, Open Source, Free

📊 Updates:
- Stats section: 3 → 4 Tools
- Footer: Added Stirling link
- README: Added tool description
- Grid layout: Now displays 4 tools responsively

🔗 URL: https://stirling.kit.pivoine.art

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

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

components/ToolsGrid.tsx

@@ -43,6 +43,19 @@ const tools = [
       </svg>
     ),
   },
+  {
+    title: 'Stirling',
+    description: 'Powerful locally-hosted PDF toolkit with 50+ operations. Merge, split, convert, OCR, sign, and manipulate PDFs with complete privacy.',
+    url: 'https://stirling.kit.pivoine.art',
+    gradient: 'gradient-purple-blue',
+    badges: ['Privacy', 'Open Source', '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 21h10a2 2 0 002-2V9.414a1 1 0 00-.293-.707l-5.414-5.414A1 1 0 0012.586 3H7a2 2 0 00-2 2v14a2 2 0 002 2z" />
+        <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M10 12h4m-4 4h4" />
+      </svg>
+    ),
+  },
 ];
 
 export default function ToolsGrid() {