[sheets] The default sheet name should be `default`

The default sheet name on newly created spreadsheets should be `default` instead of `data`. This aligns with the Helix behaviour of `default` sheet for multi sheets.

The issue identifies newly created spreadsheets and the current default sheet name, but it mentions no file or test. Search the JavaScript code for the `data` sheet name and the spreadsheet creation entry point, then verify that new spreadsheets use `default` while preserving the Helix multi-sheet behavior.

Written by the indexing model from the issue text.