Add support for 'page' as a special shapeId value in the MCP export_shape
tool. It resolves to penpot.root, exporting the entire current page as a
PNG or SVG snapshot.
Previously only 'selection' and explicit shape IDs were supported. The new
'page' shortcut is useful for AI agents needing a bird's-eye view of the
design without having to know a specific shape ID.
Closes https://github.com/penpot/penpot/issues/8689
Signed-off-by: Abhishek Mittal <abhishekmittaloffice@gmail.com>