Add modern CSS template with responsive design

[Copilot]

Implements a clean, modern template to replace the minimal default styling.

Changes

• CSS stylesheet (assets/css/style.css)

  • CSS variables for color scheme (primary: #2c3e50, secondary: #3498db, accent: #e74c3c)
  • Responsive breakpoints at 768px and 480px
  • System font stack for modern typography
  • Component styles: sticky header, hero section, post cards, footer

• Layout templates

  • default.html / supertab.html: Added header with navigation, footer
  • post.html: New layout for blog posts with metadata display
  • Posts updated to use post layout instead of default

• Homepage (index.md)

  • Hero section with gradient background
  • Post list styled as cards with hover effects

Screenshots

Desktop

Blog Post

Mobile

Notes

• About link is placeholder (no corresponding page exists yet)
• Supertab paygate.js integration maintained in both layouts

Original prompt

Add a nice template so the site looks clean and modern

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