fix: raise settings panel z-index so shadow renders above nodes

Bump ControlPanel from z-[50] to z-[90] so the layered box-shadow
paints in front of the React Flow canvas stacking context, not
behind it. Still below z-[100] used by modals and toolbars.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/nodes/ControlPanel.tsx

@@ -116,7 +116,7 @@ export function ControlPanel() {
   }
 
   return (
-    <div className="fixed top-0 right-6 h-screen z-[50] flex items-center pointer-events-none">
+    <div className="fixed top-0 right-6 h-screen z-[90] flex items-center pointer-events-none">
       <div
         className="w-80 bg-neutral-800 border border-neutral-700 rounded-xl max-h-[80vh] overflow-y-auto pointer-events-auto transition-opacity duration-200 nowheel"
         style={{