style(logo): update DemoBadge color scheme to darker theme

Changed the DemoBadge component's color classes from lighter to darker shades (text: brand-50 to brand-500, background: brand-200 to brand-950, border: brand-100 to brand-900) for better contrast and alignment with the overall dark theme.

Author: CharlyMartin

src/components/ui/logo.tsx

@@ -9,7 +9,7 @@ function Logo() {
 
 function DemoBadge() {
   return (
-    <span className="uppercase px-2 py-0.5 rounded-sm text-brand-50 border bg-brand-200 text-sm font-mono border-brand-100">
+    <span className="uppercase px-2 py-0.5 rounded-sm text-brand-500 border bg-brand-950 text-sm font-mono border-brand-900">
       Demo
     </span>
   )