Add dark/light mode toggle

## Description

The app currently uses a dark theme. Add a toggle to switch between dark and light mode, with the preference saved in localStorage. ## Requirements

• Add a theme toggle button in the Navbar (sun/moon icon)
• Persist theme choice in localStorage
• Apply theme via Tailwind CSS dark mode classes
• Default to system preference if no choice is saved ## Files to Modify
• \`src/components/layout/Navbar.tsx\` — add toggle button
• \`src/app/layout.tsx\` — apply theme class to \` \`
• Crea