Installable as an app (PWA): manifest, pixel-art icons, offline service worker

- manifest.webmanifest (full screen, any orientation), icons 192/512/maskable/apple-touch
- sw.js: network first, cache as offline fallback, the savegame API is never cached
- registered only over HTTPS or on localhost; nginx serves the manifest type
- README: installing, HTTPS requirement

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

README.md

@@ -21,6 +21,12 @@ docker compose up -d --build
 # open http://localhost:18093
 ```
 
+### Install as an app (PWA)
+
+DungeonCraft is a progressive web app: browsers offer to install it (Chrome/Edge: install icon in the address bar or "Install app" in the menu; Android: "Add to home screen"; iOS Safari: Share → "Add to Home Screen"). The installed game starts in full screen and also works offline (the savegame list on the server then falls back to savegame files).
+
+Browsers only allow the installation over **HTTPS** (or on `localhost`), so a public installation needs a reverse proxy with a certificate in front of port 18093.
+
 ### Building without Docker
 
 Requirements: Rust with the `wasm32-unknown-unknown` target.