fix: add defensive guard for answer nodes in session navigation

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

frontend/src/pages/TreeNavigationPage.tsx

@@ -757,6 +757,15 @@ export function TreeNavigationPage() {
 
       {/* Current Node */}
       <div className="bg-card border border-border rounded-xl p-6 shadow-sm">
+        {/* Answer placeholder guard */}
+        {currentNode && currentNode.type === 'answer' && (
+          <div className="rounded-lg border border-yellow-500/30 bg-yellow-500/10 p-6 text-center">
+            <p className="text-sm font-medium text-yellow-400">
+              This tree contains an unresolved placeholder node. Please contact the tree author to complete it before use.
+            </p>
+          </div>
+        )}
+
         {/* Decision Node */}
         {currentNode && currentNode.type === 'decision' && (
           <>