`session_id`: session to use for availability and metadata. If omitted, only global availability applies and product_id must be set when checkout is on.
`product_id`: WooCommerce product when checkout is enabled and no session product override exists.
`theme`: `light` (default) or `dark`.
`show_date` / `show_time`: toggle calendar and slot list.
`show_form`: `1` to show inline customer fields; `0` to rely on checkout/customer data.