Add documentation about the test account used for publishing markdown test documents, including the npub and profile link to Marky Markdown Testerson's writings.
- Add VITE_DEFAULT_ARTICLE_NADDR env variable support
- Create .env with default article naddr
- Create .env.example for documentation
- Add vite-env.d.ts for TypeScript type support
- Fallback to hardcoded value if env var not set
- Using Vite's built-in env variable support (no dotenv needed)