Add dark mode support and theme toggle

[simonrueba]

This commit introduces full dark mode support by integrating the next-themes library, providing a user-accessible theme toggle, and updating UI components and styles to respond to light, dark, and system preferences.

• Wrap the application in ThemeProvider (next-themes) with system preference enabled
• Create a ThemeToggle component using Radix DropdownMenu for selecting Light, Dark, or System themes
• Install and configure the next-themes dependency in package.json
• Update global CSS to apply dark mode colors via Tailwind’s dark: variants
• Adjust Tailwind config to use class-based dark mode (darkMode: "class")
• Add dark-specific styling to EncoderSelect, TokenViewer, and index.tsx components

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