The only callers of `menus::menu_open` are the native palette-window
code in `panel_docking.rs` and its native tests, so on wasm32 the
re-export and the function were unused and `cargo xtask wasm` printed
two warnings for them. Both now carry `#[cfg(not(target_arch = "wasm32"))]`.
The menu bar still records whether a menu is open on every target.