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>

README.md

@@ -135,6 +135,7 @@ Tailwind CSS 4 uses a new CSS-first configuration approach:
 - **Paint** - Browser-based image editor
 - **Pastel** - Modern color manipulation toolkit with palette generation and accessibility testing
 - **Stirling** - Powerful PDF toolkit with 50+ operations (merge, split, convert, OCR, sign)
+- **Units** - Smart unit converter with 187 units across 23 categories (length, mass, temperature, etc.)
 
 ## CI/CD Pipeline