feat: Upgrade to Tailwind CSS v4.1.18

- Migrate from tailwindcss v3.3 to v4.1.18
- Replace @tailwind directives with @import 'tailwindcss'
- Move custom colors to @theme block in globals.css
- Convert custom utilities to @utility syntax
- Update PostCSS config to use @tailwindcss/postcss
- Remove autoprefixer (now built-in)

src/components/OnboardingTutorial.tsx

@@ -110,7 +110,7 @@ export default function OnboardingTutorial() {
                 initial={{ opacity: 0 }}
                 animate={{ opacity: 1 }}
                 exit={{ opacity: 0 }}
-                className="fixed inset-0 z-[200] bg-black/90 backdrop-blur-sm flex items-center justify-center p-6"
+                className="fixed inset-0 z-200 bg-black/90 backdrop-blur-xs flex items-center justify-center p-6"
             >
                 {/* Close button */}
                 <button