T44d Web · Gallery builds for wasm32: one story per section by URL, themed by the host

The gallery's entry runs natively or, on wasm32, from a wasm_bindgen start
through gpui_web, single-threaded with WebGL2 fallback. ?page, ?story and
?theme choose what shows; an unknown one fails in the page and the log.
set_theme switches the mode later. Pictures load from ELY_GALLERY_ASSETS.
scripts/stories.py writes examples/gallery/stories.json from the pages.

examples/gallery/pages/layout/mod.rs

@@ -7,7 +7,7 @@ mod systems;
 use gpui::{AnyElement, App, IntoElement, ParentElement, Window, div};
 
 use super::Page;
-use crate::script::Step;
+use crate::step::Step;
 
 pub const PAGE: Page = Page {
     number: 3,