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/theme/custom.js

@@ -0,0 +1,17 @@
+// Pivoine Rose Custom Theme Injector
+(function() {
+  // Inject custom CSS
+  var customCSS = document.createElement('link');
+  customCSS.rel = 'stylesheet';
+  customCSS.href = '/css/custom.css';
+  document.head.appendChild(customCSS);
+
+  // Inject custom favicon
+  var customFavicon = document.createElement('link');
+  customFavicon.rel = 'icon';
+  customFavicon.type = 'image/svg+xml';
+  customFavicon.href = '/images/favicon-custom.svg';
+  document.head.appendChild(customFavicon);
+
+  console.log('Pivoine Rose theme loaded');
+})();