Styling updates en sign-in component

- Shadcn UI design tokens toevoegen aan globals.css
- Dark mode CSS variabelen voor consistente theming
- AnimatedSignIn component met collage layout
- Theme toggle en Google OAuth placeholder

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

tailwind.config.ts

@@ -1,7 +1,7 @@
 import type { Config } from 'tailwindcss'
 
 const config: Config = {
-  darkMode: 'class',
+  darkMode: ['class', 'class'],
   content: [
     './pages/**/*.{js,ts,jsx,tsx,mdx}',
     './components/**/*.{js,ts,jsx,tsx,mdx}',