fix: cast tree_type to TreeType in session-to-flow creation

Fixes build error where string was not assignable to TreeType.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/pages/SessionDetailPage.tsx

@@ -259,7 +259,7 @@ export function SessionDetailPage() {
       const tree = await treesApi.create({
         name: flowData.name,
         description: flowData.description,
-        tree_type: flowData.tree_type,
+        tree_type: flowData.tree_type as import('@/types').TreeType,
         tree_structure: flowData.tree_structure,
         tags: flowData.tags,
       })