fix: add assets/ directory to Docker image

The game PNG assets (maps, characters, NPCs) were not being copied
into the nginx container, causing 404s in production.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Dockerfile

@@ -2,4 +2,5 @@ FROM nginx:alpine
 COPY index.html /usr/share/nginx/html/
 COPY css/ /usr/share/nginx/html/css/
 COPY js/ /usr/share/nginx/html/js/
+COPY assets/ /usr/share/nginx/html/assets/
 EXPOSE 80