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/Stats.tsx

@@ -4,7 +4,7 @@ import { motion } from 'framer-motion';
 
 const stats = [
   {
-    number: '3',
+    number: '4',
     label: 'Tools',
     icon: (
       <svg className="w-6 h-6" fill="none" stroke="currentColor" viewBox="0 0 24 24">