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/LanguageSwitcher.tsx

@@ -11,7 +11,7 @@ const LanguageSwitcher = () => {
             <button
                 onClick={() => setLocale('de')}
                 className={`p-1.5 rounded-lg transition-all ${locale === 'de'
-                    ? 'bg-orange-950/30 scale-110 shadow-sm shadow-orange-950/20'
+                    ? 'bg-orange-950/30 scale-110 shadow-xs shadow-orange-950/20'
                     : 'opacity-50 hover:opacity-100 grayscale hover:grayscale-0'
                     }`}
                 title="Deutsch"
@@ -21,7 +21,7 @@ const LanguageSwitcher = () => {
             <button
                 onClick={() => setLocale('en')}
                 className={`p-1.5 rounded-lg transition-all ${locale === 'en'
-                    ? 'bg-orange-950/30 scale-110 shadow-sm shadow-orange-950/20'
+                    ? 'bg-orange-950/30 scale-110 shadow-xs shadow-orange-950/20'
                     : 'opacity-50 hover:opacity-100 grayscale hover:grayscale-0'
                     }`}
                 title="English"