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/components/session/SharedSessionTreePreview.tsx

@@ -80,7 +80,7 @@ export function SharedSessionTreePreview({
         <h3 className="text-sm font-semibold text-white">Tree Structure</h3>
       </div>
       <div className="max-h-[600px] overflow-y-auto py-2">
-        <TreeNode node={treeStructure} depth={0} pathTaken={pathTaken} />
+        <TreeNode node={treeStructure as unknown as Record<string, unknown>} depth={0} pathTaken={pathTaken} />
       </div>
     </div>
   )