chore: Document offline support, incl. offline_enabled attr for web component

Author: grega

README.md

@@ -91,6 +91,7 @@ The `editor-wc` tag accepts the following attributes, which must be provided as
 - `output_split_view`: Start with split view in output panel (defaults to `false`, i.e. tabbed view)
 - `project_name_editable`: Allow the user to edit the project name in the project bar (defaults to `false`)
 - `react_app_api_endpoint`: API endpoint to send project-related requests to
+- `offline_enabled`: Show an offline indicator when the user's device loses connectivity (defaults to `false`). Requires the service worker to be registered on the host page — see [Offline support](#offline-support).
 - `read_only`: Display the editor in read only mode (defaults to `false`)
 - `sense_hat_always_enabled`: Show the Astro Pi Sense HAT emulator on page load (defaults to `false`)
 - `show_save_prompt`: Prompt the user to save their work (defaults to `false`)
@@ -130,6 +131,33 @@ The host page is able to communicate with the web component via custom methods p
 
 This allows the host page to query the current code in the editor and to control code runs from outside the web component, for example.
 
+### Offline support
+
+The web component ships a service worker (`service-worker.js`) that caches the editor shell and Pyodide assets so the component remains usable after a network loss.
+
+To enable offline support on your host page:
+
+1. Register the service worker from your host page (or let the bundled `web-component.html` do it automatically):
+   ```js
+   if ("serviceWorker" in navigator) {
+     navigator.serviceWorker.register("./service-worker.js");
+   }
+   ```
+2. Pass the `offline_enabled` attribute to the web component so the offline indicator is shown when connectivity is lost:
+   ```html
+   <editor-wc offline_enabled="true"></editor-wc>
+   ```
+
+Offline mode is opt-in — neither the service worker registration nor the offline badge will appear unless these steps are taken.
+
+#### Developing with offline support
+
+The service worker is not registered in development by default. To enable it locally, set the environment variable before starting the dev server:
+
+```sh
+REACT_APP_ENABLE_SERVICE_WORKER=true yarn start
+```
+
 ## Development
 
 ### Previewing

src/web-component.html

@@ -109,6 +109,8 @@
         newWebComp.setAttribute("with_projectbar", "true");
         newWebComp.setAttribute("with_sidebar", "true");
         newWebComp.setAttribute("use_editor_styles", "true");
+        // see "Offline support" section in README for more details
+        newWebComp.setAttribute("offline_enabled", "true");
         newWebComp.setAttribute(
           "sidebar_options",
           JSON.stringify([