feat: create beautiful ShadCN-inspired dark theme from scratch

- Build completely custom dark theme inspired by ShadCN design system
- Use CSS custom properties for consistent theming
- Create elegant dark color palette with proper contrast
- Add beautiful typography and spacing
- Implement smooth transitions and hover effects
- Add custom scrollbar styling
- Create responsive design that works on all devices
- Ensure sidebar navigation persists across all pages
- Add focus styles for accessibility
- Build from ground up with no external theme dependencies

Author: rainmana