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/FloatingScannerButton.tsx

@@ -108,7 +108,7 @@ export default function FloatingScannerButton({ onImageSelected }: FloatingScann
                                 ease: "easeInOut",
                                 repeatDelay: 3
                             }}
-                            className="absolute inset-0 bg-gradient-to-r from-transparent via-white/40 to-transparent skew-x-12 -z-0"
+                            className="absolute inset-0 bg-linear-to-r from-transparent via-white/40 to-transparent skew-x-12 z-0"
                         />
                     )}