Make the web app work offline and update itself (#239)

AGENTS.md

@@ -75,7 +75,10 @@ People trust SolidCraft with their designs; a crash loses their work. **This out
   egui/eframe/winit/wgpu/rfd. **The UI crate is swappable.** Only `crates/kernel` may use `truck`.
 - **The UI is thin**: panels read engine state and act through `app.run(id, params)` /
   `app.start(id)`. Colours come from `theme::Tokens`.
-- **Rust only.** Units are millimetres and radians internally; Z is up.
+- **Rust only.** Units are millimetres and radians internally; Z is up. The one exception is
+  JavaScript the browser requires: the service worker `apps/solidcraft-web/sw.js` (browsers only
+  run service workers as JavaScript) and the small inline scripts in `apps/solidcraft-web/index.html`.
+  Keep them minimal; `cargo xtask web-offline-test` covers the service worker.
 - **Quality gates** before every commit: `cargo xtask ci` (fmt, clippy -D warnings, tests,
   assets, layers). Commit after every feature arc that builds.
 - **Publishing**: `origin` is github.com/bherbruck/solidcraft. Changes land through **pull requests**: