feat(pwa): make the app installable

Manifest, gamepad icon (also the new favicon, instead of
the Vite logo), iOS meta tags and the tab title
ToBePlayed. No service worker, so a deploy never meets
an old cached bundle. The title bar color follows
light and dark mode. The README also lists adding a
game by name.

nginx.conf

@@ -7,6 +7,12 @@ server {
         add_header Cache-Control "public, max-age=31536000, immutable";
     }
 
+    # Not in every nginx mime.types; browsers expect this type for the PWA manifest.
+    location = /manifest.webmanifest {
+        types { application/manifest+json webmanifest; }
+        add_header Cache-Control "no-cache";
+    }
+
     # index.html and /i18n/*.json keep their names across deploys. Without
     # no-cache the browser shows old translation keys or loads an old bundle.
     location / {