Full-Stack Landing Page Code Generator (HTML/CSS/JS)

You are an expert front-end developer who writes clean, modern, conversion-optimized landing pages.

Generate a complete, production-ready landing page for the following product or service:

- Product/Service name: {product_name}
- What it does (one sentence): {product_description}
- Target audience: {target_audience}
- Main CTA (call to action): {main_cta} (e.g., "Start Free Trial", "Book a Demo", "Download Now")
- Color palette / brand style: {brand_style} (e.g., "dark SaaS, blue/purple gradient" or "minimal, white with green accents")
- Sections to include: {sections} (e.g., Hero, Features, Pricing, Testimonials, FAQ, Footer)

Requirements:
- Write the full HTML in a single file with embedded CSS (using CSS variables for the color palette) and vanilla JavaScript
- Use a mobile-first, responsive design with CSS Grid and Flexbox
- Include smooth scroll, a sticky navbar, and a scroll-triggered animation (fade-in on scroll using IntersectionObserver)
- The hero section must have a compelling headline, subheadline, and a prominent CTA button
- Add a features section with 3–6 feature cards using icons (use inline SVG or emoji icons)
- If pricing is requested, show 3 tiers (Free, Pro, Enterprise)
- Ensure the code is semantic HTML5, accessible (ARIA labels), and optimized for Core Web Vitals
- Add a comment block at the top explaining the structure

Output only the complete, copy-paste-ready HTML file. No explanations outside the code.