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/mod.rs

@@ -44,7 +44,7 @@ mod typography;
 
 use gpui::{AnyElement, App, Window};
 
-use crate::script::Step;
+use crate::step::Step;
 
 pub struct Page {
     pub number: u8,
@@ -53,6 +53,7 @@ pub struct Page {
     pub summary: &'static str,
     pub render: fn(&mut Window, &mut App) -> AnyElement,
     /// Interactions shot after the static pages.
+    #[cfg_attr(target_family = "wasm", allow(dead_code))]
     pub script: &'static [Step],
 }