Redesign playground: template tabs, live preview, split pane, mobile responsive

- Add 3 pre-built templates (Invoice, Report, Custom HTML)
- Split-pane editor with live HTML preview (updates as you type)
- Generation timer shows actual response time
- Before/after comparison (free watermarked vs Pro clean)
- Pro CTA integrated into result panel
- Fully responsive: stacks on mobile
- Professional polish matching site design language

public/docs.html

@@ -14,7 +14,6 @@
   <meta name="twitter:card" content="summary_large_image">
   <meta name="twitter:title" content="DocFast API Documentation">
   <meta name="twitter:description" content="Convert HTML and Markdown to PDF with a simple REST API call.">
-  <meta name="twitter:image" content="https://docfast.dev/og-image.png">
   <link rel="icon" href="data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 100 100'><text y='.9em' font-size='90'>⚡</text></svg>">
   <link rel="stylesheet" href="/swagger-ui/swagger-ui.css">
   <style>