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/ActiveSessionBanner.tsx

@@ -19,7 +19,7 @@ export default function ActiveSessionBanner() {
                     initial={{ y: 50, opacity: 0, x: '-50%' }}
                     animate={{ y: 0, opacity: 1, x: '-50%' }}
                     exit={{ y: 50, opacity: 0, x: '-50%' }}
-                    className="fixed bottom-32 left-1/2 z-[50] w-[calc(100%-2rem)] max-w-sm"
+                    className="fixed bottom-32 left-1/2 z-50 w-[calc(100%-2rem)] max-w-sm"
                 >
                     <div className="bg-zinc-900/90 backdrop-blur-2xl border border-orange-500/20 rounded-[32px] p-2 flex items-center justify-between shadow-2xl ring-1 ring-white/5 overflow-hidden">
                         {/* Session Info Link */}