feat: add dark mode with system preference + manual toggle
- CSS custom properties for dark theme on [data-theme="dark"] and prefers-color-scheme
- Theme toggle button (☀/🌙) in navbar, persisted to localStorage
- Flash-free theme application via inline script in <head>
- CodeMirror oneDark theme switches dynamically via Compartment
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>