Redesign main page with minimalist "No" and move interactive UI to /playground

- Create elegant minimalist main page with centered "No"
- Add automatic dark mode support for main page
- Move original interactive API testing interface to /playground
- Add footer links to main page for navigation
- Update routing to serve both pages appropriately
- Update documentation to reflect new page structure

The main page now provides a clean, focused experience while the
playground remains available for API testing and exploration.

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

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

README.md

@@ -14,7 +14,8 @@ A lightweight, production-ready HTTP service that always says "no" in various fo
 
 ## API Endpoints
 
-- `GET /` - Web frontend
+- `GET /` - Minimalist web page displaying "No"
+- `GET /playground` - Interactive API testing playground
 - `GET /api/no` - Returns plain text "no"
 - `GET /api/no?format=json` - Returns `{"answer": "no"}`
 - `GET /api/no?format=bool` - Returns `false`