[FEAT] Implement global dark mode with toggle, persistence, and OS preference sync

[Siddh2024]

Fixes #304

Description

Implemented a comprehensive dark mode across the entire platform with CSS custom properties for seamless theming.

Features

• Global dark mode applied consistently across all pages
• Theme toggle button in the navbar
• localStorage persistence (remembers user choice)
• OS preference sync (defaults to system preference)
• Smooth transitions between themes
• CSS variables for easy maintenance

Files Changed

• Frontendd/src/index.css - Added smooth transition CSS for theme switching
• Frontendd/src/App.jsx - Removed redundant darkMode state management (now handled by ThemeContext)
• Frontendd/src/components/mvpblocks/header-2.jsx - Fixed hardcoded white backgrounds in mobile menu to use CSS variables
• Frontendd/src/components/mvpblocks/footer-standard.jsx - Replaced hardcoded slate/gray colors with theme variables
• Frontendd/src/components/ui/confirmation-modal.jsx - Updated to use card/theme colors
• Frontendd/src/components/ui/legal-modal.jsx - Updated to use card/muted/theme colors
• Frontendd/src/components/ui/ScrollToTop.jsx - Updated to use foreground/background variables
• Frontendd/src/components/CoOrganizerPanel.jsx - Replaced hardcoded gray colors with theme variables
• Frontendd/src/pages/SignIn.jsx - Replaced hardcoded white/gray colors with theme variables
• Frontendd/src/pages/SignUp.jsx - Replaced hardcoded white/gray colors with theme variables
• Frontendd/src/pages/ForgotPassword.jsx - Updated dark-themed card to use CSS variables

Impact

• Reduced eye strain for night-time users
• Modern, professional appearance
• Accessible theme switching
• Seamless integration with existing Tailwind CSS variables

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