feat: inject custom CSS theme into asciinema

Added entrypoint wrapper script that injects custom.css into the
main app.css file at container startup. This allows the custom
Pivoine theme to be applied without building a custom image.

Changes:
- Mount custom.css to static assets directory
- Add entrypoint-wrapper.sh to inject CSS on startup
- Append custom CSS to both app.css and hashed app-*.css

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

asciinema/compose.yaml

@@ -3,13 +3,16 @@ services:
     image: ${ASCIINEMA_IMAGE:-ghcr.io/asciinema/asciinema-server:latest}
     container_name: ${ASCIINEMA_COMPOSE_PROJECT_NAME}_app
     restart: unless-stopped
+    entrypoint: ["/entrypoint-wrapper.sh"]
+    command: ["start"]
     networks:
       - compose_network
     extra_hosts:
       - "smtp.ionos.de:213.165.67.97"
     volumes:
       - asciinema_data:/var/opt/asciinema
-      - ./theme/custom.css:/app/assets/css/custom.css:ro
+      - ./theme/custom.css:/opt/app/lib/asciinema-1.0.0/priv/static/assets/custom.css:ro
+      - ./entrypoint-wrapper.sh:/entrypoint-wrapper.sh:ro
     environment:
       SECRET_KEY_BASE: ${ASCIINEMA_SECRET_KEY}
       URL_HOST: ${ASCIINEMA_TRAEFIK_HOST}