chore: remove custom theme customization

Remove all theme customization code:
- Removed theme directory (custom.css, custom.js, favicon.svg)
- Removed app.html.heex template override
- Removed Dockerfile for custom build
- Reverted compose.yaml to only mount custom.exs for SMTP config

Keeping only SMTP configuration in custom.exs.

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

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

asciinema/compose.yaml

@@ -8,10 +8,6 @@ services:
     volumes:
       - asciinema_data:/var/opt/asciinema
       - ./custom.exs:/opt/app/etc/custom.exs:ro
-      - ./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}