feat: mount custom theme template to inject CSS and favicon

Mount app.html.heex template with custom theme links:
- Custom CSS at /assets/custom.css via Routes.static_path()
- Custom favicon at /images/favicon-custom.svg

Phoenix will use the external template file instead of compiled version.

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

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

asciinema/compose.yaml

@@ -11,6 +11,7 @@ services:
       - ./theme/custom.css:/opt/app/lib/asciinema-1.0.0/priv/static/assets/custom.css:ro
       - ./theme/favicon.svg:/opt/app/lib/asciinema-1.0.0/priv/static/images/favicon-custom.svg:ro
       - ./theme/custom.js:/opt/app/lib/asciinema-1.0.0/priv/static/js/custom.js:ro
+      - ./app.html.heex:/opt/app/lib/asciinema-1.0.0/lib/asciinema_web/templates/layout/app.html.heex:ro
     environment:
       SECRET_KEY_BASE: ${ASCIINEMA_SECRET_KEY}
       URL_HOST: ${ASCIINEMA_TRAEFIK_HOST}