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>

nginx.conf

@@ -7,6 +7,11 @@ server {
     location / {
         try_files $uri $uri/ =404;
     }
+    # installable app (PWA)
+    location ~ \.webmanifest$ {
+        default_type application/manifest+json;
+        try_files $uri =404;
+    }
     # savegame service (server/). Resolved at request time, so the game still starts without it;
     # the game then falls back to savegame files.
     location /api/ {