feat: Implement dark mode for accessibility

Relates to issue #7

Changes

New Files Created

• static/css/dark-mode.css - Complete dark mode stylesheet with CSS custom properties for consistent theming, WCAG AA compliant contrast ratios (4.5:1), and comprehensive coverage for all UI elements including header, navigation, footer, cards, buttons, forms, product grid, cart items, and checkout form
• static/js/theme-toggle.js - Theme toggle functionality with localStorage persistence, system preference detection, automatic theme switching, smooth transitions, and full accessibility support with ARIA attributes

Modified Files

• templates/base.html - Added color-scheme meta tag, dark-mode.css stylesheet link, theme toggle button in navigation with accessibility attributes, and theme-toggle.js script
• static/css/style.css - Added CSS custom property for transition speed and smooth transitions for background-color and color changes, plus theme toggle button styles

Key Features

✅ Automatic theme detection based on system preferences
✅ Manual theme toggle with persistent localStorage storage
✅ Smooth transitions between light and dark modes (0.3s ease)
✅ Dynamic icon updates (moon/sun) based on current theme
✅ Full accessibility support with ARIA attributes and WCAG AA contrast compliance
✅ System preference change listener for automatic updates
✅ Comprehensive dark mode coverage for all pages including home, products, cart, checkout, and demo pages

User Experience

• Users can toggle dark mode by clicking the theme button in the navigation
• Theme preference is saved and persists across sessions
• If no manual preference is set, the app respects system dark mode settings
• Smooth visual transitions prevent jarring theme switches
• All interactive elements maintain proper contrast and visibility in both modes