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/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">3 Tools</span>
+              <span className="text-xs font-medium text-purple-400">4 Tools</span>
               <span className="text-gray-600">•</span>
               <span className="text-xs text-gray-500">Open Source</span>
             </div>
@@ -63,6 +63,7 @@ export default function Footer() {
               <a href="https://vert.kit.pivoine.art" className="text-gray-400 hover:text-green-400 transition-colors text-sm">Vert</a>
               <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>
             </div>
           </div>
         </motion.div>