Offline downloads, service worker, iOS audio unlock fix

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

README.md

@@ -32,3 +32,15 @@ PLAYER_PASSWORD=...
 
 `docker compose up -d`, then sign in at port 8080. The default account is only created if it doesn't exist, so changing its password later with
 `docker exec -it audiobook-player python server.py passwd nate` sticks. Add more people with `adduser`.
+
+## Offline listening
+
+Tap the ↓ on a book (or "Download" in the player) to save it on the device. Downloaded books show a ✓, and the
+"Downloaded only" view lists them. They play with no connection to the server, and progress syncs when you're back online.
+
+- Keep the app open while a download runs; iOS pauses pages in the background.
+- The audio is stored in the browser (IndexedDB). iOS counts it against the site's storage quota, which is much larger
+  once the site is added to the Home Screen.
+- **To open the app itself offline, serve it over HTTPS.** Browsers only allow the service worker (`sw.js`) on secure
+  origins. Put a reverse proxy with a certificate in front of port 8080 (and a domain name). Over plain HTTP, downloads
+  still play but the page can't load without a connection.