feat: enhance documentation and design system components
- Rewrite API.md and EXAMPLES.md with a new structure and more comprehensive guides.
- Introduce a themed component system including StandardSiteLayout, ThemedCard, and ThemeToggle.
- Add themeStore and theme utilities for managing light/dark modes and dynamic CSS variables.
- Update DocumentCard and PublicationCard to support themes and Lucide icons.
- Add @lucide/svelte as a dependency for enhanced UI elements.
- Export new style entry points for base and theme CSS.