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>

CLAUDE.md

@@ -56,10 +56,13 @@ The entire application is contained in `src/main.rs` with ~372 lines of code. Th
 
 ### Key Components
 
-1. **Embedded HTML Frontend**: The HTML/CSS/JavaScript frontend is embedded as a string constant (`HTML_FRONTEND`) directly in the source code, eliminating the need for static file serving.
+1. **Embedded HTML Frontends**: Two HTML frontends are embedded as string constants:
+   - `SIMPLE_FRONTEND`: Minimalist main page with just "No" centered on screen
+   - `PLAYGROUND_FRONTEND`: Interactive API testing playground with buttons and examples
 
 2. **Request Router**: The `route_request()` function handles all routing logic:
-   - `/` - Serves the embedded HTML frontend
+   - `/` - Serves the minimalist "No" page
+   - `/playground` - Serves the interactive API testing interface
    - `/api/no` - API endpoint with format parameter support
    - `/health` - Health check endpoint
    - CORS headers are added to all responses