feat: add dark mode toggle with system preference detection

[yeap531]

• Add @nuxtjs/color-mode module for theme management
• Create ThemeToggle component with sun/moon icons
• Add dark: variants to all components (layout, navbar, sidebar, etc.)
• Support system preference detection and localStorage persistence
• Smooth transition animations between themes

Closes #2489

ℹ️ Repository information

The repository has:

• [ ] At least three issues with the good first issue label.
• [ ] At least 10 contributors.
• [ ] Detailed setup instructions for the project.
• [ ] CONTRIBUTING.md
• [ ] Actively maintained.

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