feat: add custom Pivoine Rose theme to asciinema

- Mount custom CSS, favicon, and JS into static directories
- Create custom.js to inject theme CSS and favicon via JavaScript
- Add CustomThemeInjector Plug in custom.exs to inject script tag
- Custom theme features:
  - Pivoine rose primary color (#CE275B)
  - Gray tone backgrounds
  - Custom SVG favicon with rose gradient
  - Bootstrap 4 component overrides

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

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

asciinema/compose.yaml

@@ -8,8 +8,9 @@ services:
     volumes:
       - asciinema_data:/var/opt/asciinema
       - ./custom.exs:/opt/app/etc/custom.exs:ro
-      - ./theme:/opt/app/priv/static/theme:ro
-      - ./app.html.heex:/opt/app/lib/asciinema_web-*/eex/layout/app.html.heex:ro
+      - ./theme/custom.css:/opt/app/priv/static/css/custom.css:ro
+      - ./theme/favicon.svg:/opt/app/priv/static/images/favicon-custom.svg:ro
+      - ./theme/custom.js:/opt/app/priv/static/js/custom.js:ro
     environment:
       SECRET_KEY_BASE: ${ASCIINEMA_SECRET_KEY}
       URL_HOST: ${ASCIINEMA_TRAEFIK_HOST}