Improve explorer refresh cursor restore

[Maxteabag]

Summary

• Switch explorer refresh to an incremental rebuild to reduce flicker
• Restore cursor + expansion state during refresh, including async folder/column loads
• Add tests for cursor restore and expanded-node loading
• Add DuckDB integration test for manual refresh expansion

Testing

• pytest -q tests/ui/explorer/test_refresh_cursor_restore.py
• pytest -q tests/ui/explorer/test_refresh_expanded_loads.py

Notes

• DuckDB integration test not re-run after latest change