Add dark mode toggle and yellow gradient background

[devloai]

🎨 Changes

This PR introduces a dark mode toggle and updates the background gradient to yellow tones.

Features Added

• ✨ Dark Mode Toggle: Interactive theme switcher in navbar with sun/moon icons
• 🎨 Yellow Gradient Background: Warm yellow gradient (yellow-50 → yellow-200) in light mode
• 🌙 Dark Mode Support: Complete dark theme with gray gradient background
• 💾 Theme Persistence: User preference saved and persists across sessions
• ⚡ System Theme Support: Respects OS-level dark mode preference

Implementation

• Installed next-themes package for theme management
• Added ThemeProvider component wrapping the app
• Created ThemeToggle component with smooth transitions
• Updated all components with dark: Tailwind classes:
  • Background gradients
  • Navbar and footer
  • Cards and text elements
  • Buttons and interactive elements
• Enabled darkMode: "class" in Tailwind config

Screenshots

Light Mode (Yellow Gradient)

Dark Mode (Gray Gradient)

Preview

🔗 Preview Link

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Testing: Toggle button in navbar works seamlessly, theme persists on page reload, all UI elements properly adapt to both themes.