fix: make master mute button square (48x48px)

Changed button dimensions from h-8 w-full to h-12 w-12 for a
square/quadratic shape matching the design of track buttons.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

components/controls/MasterControls.tsx

@@ -72,7 +72,7 @@ export function MasterControls({
       <button
         onClick={onMuteToggle}
         className={cn(
-          'h-8 w-full rounded-md flex items-center justify-center transition-all text-[11px] font-bold',
+          'h-12 w-12 rounded-md flex items-center justify-center transition-all text-[11px] font-bold',
           isMuted
             ? 'bg-blue-500 text-white shadow-md shadow-blue-500/30'
             : 'bg-card hover:bg-accent text-muted-foreground border border-border/50'