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/BuddyHandshake.tsx

@@ -106,7 +106,7 @@ export default function BuddyHandshake({ isOpen, onClose, onSuccess }: BuddyHand
                 initial={{ opacity: 0 }}
                 animate={{ opacity: 1 }}
                 exit={{ opacity: 0 }}
-                className="fixed inset-0 bg-black/80 backdrop-blur-sm z-50 flex items-center justify-center p-4"
+                className="fixed inset-0 bg-black/80 backdrop-blur-xs z-50 flex items-center justify-center p-4"
                 onClick={onClose}
             >
                 <motion.div
@@ -240,7 +240,7 @@ export default function BuddyHandshake({ isOpen, onClose, onSuccess }: BuddyHand
                                         }
                                     }}
                                     placeholder="XXXXXX"
-                                    className="w-full text-center text-3xl font-black tracking-[0.4em] bg-zinc-950 border-2 border-zinc-800 rounded-2xl py-4 text-white placeholder:text-zinc-700 focus:outline-none focus:border-orange-500 transition-colors font-mono"
+                                    className="w-full text-center text-3xl font-black tracking-[0.4em] bg-zinc-950 border-2 border-zinc-800 rounded-2xl py-4 text-white placeholder:text-zinc-700 focus:outline-hidden focus:border-orange-500 transition-colors font-mono"
                                     maxLength={6}
                                     autoFocus
                                 />