fix: tree navigation root ID + session closure popover z-index

- TreeNavigationPage: initialize currentNodeId from tree_structure.id
  instead of hardcoded 'root' — fixes "Invalid tree structure" for trees
  with non-'root' root IDs (KB Accelerator, AI Flow Builder)
- SessionHistoryPage: add relative z-10 to session card when close
  popover is open so it renders above sibling cards

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

frontend/src/pages/SessionHistoryPage.tsx

@@ -279,7 +279,10 @@ export function SessionHistoryPage() {
           {sessions.map((session) => (
             <div
               key={session.id}
-              className="bg-card border border-border rounded-xl p-4 transition-all hover:bg-accent/50"
+              className={cn(
+                'bg-card border border-border rounded-xl p-4 transition-all hover:bg-accent/50',
+                closingSessionId === session.id && 'relative z-10'
+              )}
             >
               <div className="flex flex-col gap-3 sm:flex-row sm:items-start sm:justify-between">
                 <div className="flex-1">