Create a single-file index.html landing page for a fictional specialty coffee shop called "Tunnel Coffee".
Requirements:
- Responsive layout that works on mobile and desktop
- Dark theme
- Hero section with a headline and a call-to-action button
- Menu section with 6 items and prices
- Opening hours section
- Contact form with client-side validation in JavaScript (no backend)
- Smooth scroll navigation
- No external libraries or CDNs
Output only the complete HTML code in one file.