fix: pass treeData directly to startSession to avoid stale state

setTree(treeData) hasn't committed when startSession runs immediately
after, so tree is still null and getStepsFromTree returns []. This
caused the step detail area to render empty on new session start.

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

frontend/src/pages/ProceduralNavigationPage.tsx

@@ -185,7 +185,7 @@ export function ProceduralNavigationPage() {
 
       // Start session immediately — no intake form modal
       // Variables will be filled inline during execution
-      await startSession(id, {})
+      await startSession(id, {}, treeData)
     } catch {
       toast.error('Failed to load flow')
       navigate('/trees')
@@ -194,7 +194,7 @@ export function ProceduralNavigationPage() {
     }
   }
 
-  const startSession = async (id: string, variables: Record<string, string>) => {
+  const startSession = async (id: string, variables: Record<string, string>, treeData?: Tree) => {
     try {
       const newSession = await sessionsApi.create({
         tree_id: id,
@@ -203,9 +203,9 @@ export function ProceduralNavigationPage() {
       setSession(newSession)
       setSessionVariables(variables)
 
-      // Initialize step states
+      // Initialize step states — use passed treeData since `tree` state may not have committed yet
       const initialStates = new Map<string, StepState>()
-      const allSteps = getStepsFromTree(tree!)
+      const allSteps = getStepsFromTree(treeData || tree!)
       for (const step of allSteps) {
         initialStates.set(step.id, { notes: '', verificationValue: '', completedAt: null })
       }