fix(deploy): copy classic theme assets into Docker runtime image

solitaire_engine/assets/themes/classic/ was absent from the container
because only the workspace-root assets/ directory was copied. The
AssetServer serves themes/classic/ from that same root, so the classic
theme manifested as a missing-asset load failure at runtime.

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

solitaire_server/Dockerfile

@@ -64,6 +64,7 @@ COPY --from=builder /build/target/release/solitaire_server ./server
 #   /app/assets                →  /assets route
 COPY solitaire_server/web ./solitaire_server/web
 COPY assets ./assets
+COPY solitaire_engine/assets/themes/classic ./assets/themes/classic
 
 ENV SERVER_PORT=8080
 EXPOSE 8080