fix(a11y): add keyboard listeners to clickable elements (S1082)

Add onKeyDown (Enter/Space) to the CommandPalette overlay backdrop div and
result item divs to satisfy SonarQube S1082. RouteFlow and ProcessorTimeline
already had the fixes in place.

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

src/design-system/layout/Sidebar/SidebarTree.tsx

@@ -366,6 +366,10 @@ function SidebarTreeRow({
 
   return (
     <li role="none">
+      {/* S1082: No onKeyDown here by design — the parent <ul role="tree"> carries
+          onKeyDown={handleKeyDown} which handles Enter (navigate) and all arrow keys
+          per the WAI-ARIA tree widget pattern. Adding a duplicate handler here would
+          fire the action twice. */}
       <div
         role="treeitem"
         aria-expanded={hasChildren ? isExpanded : undefined}