Realtime broadcast for sheet structure (rename, add, delete)

[shreydevkar]

cells_changed + cursor_at sync live, but adding/renaming/deleting a sheet doesn't broadcast. Collaborators have to refresh to see the new tab.

Acceptance criteria:

• New broadcast event sheet_changed emitted on create / rename / delete / activate.
• Client handler updates workbook.sheets + tab bar.
• Active-sheet changes from a collaborator don't force-switch the peer's active sheet (just tell them a sheet changed).