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/documents/editing.rs

@@ -12,10 +12,7 @@ use crate::{
     ui::{keep, section, set},
 };
 
-const ATRIUM: &str = concat!(
-    env!("CARGO_MANIFEST_DIR"),
-    "/examples/gallery/assets/atrium.jpg"
-);
+const ATRIUM: &str = asset!("atrium.jpg");
 
 const PEOPLE: [&str; 4] = ["ada", "grace", "linus", "margaret"];