Add dark mode toggle to the app

[noeljackson]

Add a dark mode toggle button to the top-right corner of the app.

Requirements

• Toggle button switches between light and dark themes
• Use Tailwind CSS dark mode (class strategy)
• Persist preference in localStorage
• Default to system preference

Acceptance criteria

• Dark mode toggle visible in header
• Theme persists across page reloads
• Respects prefers-color-scheme on first visit

[github-actions]

Codespace launched: example-issue-3

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