Make Crystal an installable web app with an offline shell (#39)

A manifest and icons rendered from icon.svg make the app installable.
The native build emits a service worker that precaches exactly the
built files and passes everything else to the network, so ROMs never
reach its cache. New versions wait for the player's Update, which saves
before reloading. Savegames offers installation, iPhone steps and
persistent storage, and a full disk gets its own save error.

Acceptance on physical iPhone and Android devices remains.

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

README.md

@@ -43,7 +43,7 @@ npm run check:release     # Check the native distribution
 npm run build:reference   # Optional reference in dist-reference/
 ```
 
-Serve `dist/` through an HTTPS static host, or use `npm run preview` locally. Subdirectory hosting is supported with `npm run build -- --base=/Crystal/`. Use HTTPS from another device; ROM verification and save locking require a secure context.
+Serve `dist/` through an HTTPS static host, or use `npm run preview` locally. Over HTTPS the app can be installed from Savegames (or Safari's Add to Home Screen) and opens offline; select the ROM at each launch. Updates wait until you choose Update. Subdirectory hosting is supported with `npm run build -- --base=/Crystal/`. Use HTTPS from another device; ROM verification and save locking require a secure context.
 
 ## Development