feat: style NIP-05 display with muted text color

Add text-muted-foreground class to NIP-05 address display under username
in ProfileViewer for better visual hierarchy. The small font (text-xs)
and muted color make it appropriately subtle as secondary information.

src/components/ProfileViewer.tsx

@@ -398,7 +398,7 @@ export function ProfileViewer({ pubkey }: ProfileViewerProps) {
               />
               {/* NIP-05 */}
               {profile.nip05 && (
-                <div className="text-xs">
+                <div className="text-xs text-muted-foreground">
                   <Nip05 pubkey={pubkey} profile={profile} />
                 </div>
               )}