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/TreeNavigationPage.tsx

@@ -39,8 +39,8 @@ export function TreeNavigationPage() {
 
   const [tree, setTree] = useState<Tree | null>(null)
   const [session, setSession] = useState<Session | null>(null)
-  const [currentNodeId, setCurrentNodeId] = useState<string>('root')
-  const [pathTaken, setPathTaken] = useState<string[]>(['root'])
+  const [currentNodeId, setCurrentNodeId] = useState<string>('')
+  const [pathTaken, setPathTaken] = useState<string[]>([])
   const [decisions, setDecisions] = useState<DecisionRecord[]>([])
   const [currentStepEnteredAt, setCurrentStepEnteredAt] = useState<string>(new Date().toISOString())
   const [notes, setNotes] = useState<string>('')
@@ -294,7 +294,7 @@ export function TreeNavigationPage() {
         const sessionData = await sessionsApi.get(locationState.sessionId)
         setSession(sessionData)
         setPathTaken(sessionData.path_taken)
-        setCurrentNodeId(sessionData.path_taken[sessionData.path_taken.length - 1] || 'root')
+        setCurrentNodeId(sessionData.path_taken[sessionData.path_taken.length - 1] || treeData.tree_structure?.id || 'root')
         setDecisions(sessionData.decisions as DecisionRecord[])
         setCurrentStepEnteredAt(deriveCurrentStepEnteredAt(sessionData))
         customStepFlow.initCustomSteps(sessionData.custom_steps || [])
@@ -320,6 +320,10 @@ export function TreeNavigationPage() {
         client_name: clientName || undefined,
       })
       setSession(newSession)
+      // Initialize currentNodeId to the tree's actual root (may not be 'root')
+      const rootId = tree.tree_structure?.id || 'root'
+      setCurrentNodeId(rootId)
+      setPathTaken([rootId])
       setCurrentStepEnteredAt(newSession.started_at || new Date().toISOString())
       setShowMetadataForm(false)
       // Save for "Repeat Last Session"