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

docs/book/src/guide/getting-started.md

@@ -74,13 +74,22 @@ doesn't update automatically.
 
 The same app runs in the browser: [open SolidCraft on the web](../app/) (`?sample` opens the sample
 part, `?webgl` forces WebGL2 where WebGPU is missing). Designs are saved in the browser's private
-file storage, which needs HTTPS or localhost. To build it yourself:
+file storage, which needs HTTPS or localhost.
+
+After the first visit the web app works offline: the browser keeps a copy of it. Online, each
+launch gets the latest published build (the browser downloads it once when it changes); offline,
+when the network doesn't answer within a few seconds, or when a new build can't be downloaded
+whole, the kept copy starts (and the download carries on for the next launch). To build it
+yourself:
 
 ```sh
 rustup target add wasm32-unknown-unknown
-cd apps/solidcraft-web && trunk serve --release
+cd apps/solidcraft-web && trunk serve --release --no-autoreload
 ```
 
+Offline use needs `--no-autoreload`: Trunk's live reload changes the page as it serves it, so it
+no longer matches the build the browser would keep. Debug builds (`trunk serve`) never keep a copy.
+
 ## CLI
 
 `solidcraft-cli` runs the same commands without a window:

docs/releasing.md

@@ -27,6 +27,11 @@ repository under `/solidcraft/`. Do not add a separate owner-site publisher or p
 old-name Pages/Flatpak aliases. GitHub repository redirects are separate from Pages hosting;
 verify the new project URLs after deployment.
 
+The browser app keeps itself for offline use (`apps/solidcraft-web/sw.js`). Browsers keep that
+service worker after the code that registered it is gone, so to switch it off deploy
+`apps/solidcraft-web/sw.js` unbuilt (as `trunk serve` builds do): it deletes what it kept and
+unregisters. A site without `build.json` is also safe: launches then go to the network.
+
 ## Nightly prereleases
 
 `release.yml` also runs at midnight in `America/New_York` (Eastern time, following daylight