feat: Add comprehensive light theme and theme switcher

✨ Features:
- Complete light theme with improved color palette
- Theme switcher in header with sun/moon icons
- Theme persistence using localStorage
- Smooth transitions between themes

🎨 Theme Improvements:
- Softer, easier-on-eyes light theme colors
- Fixed text contrast issues across all components
- Enhanced member card and tab text readability
- Fixed hover effects that made text disappear
- Improved member overlay header and body styling

🔧 Technical Implementation:
- CSS variables system for easy theme management
- JavaScript ThemeManager class for theme switching
- Responsive design for mobile devices
- Comprehensive hover state fixes
- Z-index solutions for text visibility

📱 Components Fixed:
- Member cards (hostname, IP, latency, details)
- Navigation tabs and content
- Task summaries and progress items
- Capability forms and API endpoints
- Member overlay popup
- Form inputs and dropdowns
- Status indicators and label chips

🎯 Accessibility:
- WCAG AA contrast compliance
- Proper color hierarchy
- Clear visual feedback
- Mobile-responsive design

Files added:
- public/styles/theme.css - Theme system and variables
- public/scripts/theme-manager.js - Theme management logic
- THEME_README.md - Comprehensive documentation
- THEME_IMPROVEMENTS.md - Improvement details

Files modified:
- public/index.html - Added theme switcher and script includes
- public/styles/main.css - Updated to use CSS variables

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