fix: resolve tree_structure type compatibility for shared session views

- Use TreeStructure & Record<string, unknown> intersection for JSONB flexibility
- Add explicit cast in SharedSessionTreePreview for recursive node rendering

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

frontend/src/pages/SharedSessionPage.tsx

@@ -232,7 +232,7 @@ export function SharedSessionPage() {
           <div className="lg:col-span-2">
             <SessionTimeline
               decisions={data.decisions}
-              treeType={data.tree_structure?.tree_type}
+              treeType={data.tree_structure?.tree_type as string | undefined}
               startedAt={data.started_at}
               completedAt={data.completed_at}
               showCopyButtons={false}