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/mail/mod.rs

@@ -16,7 +16,7 @@ mod write;
 use super::Page;
 use crate::{
     probe::probe,
-    script::Step,
+    step::Step,
     ui::{change, keep, section},
 };