feat: use Phoenix config for theme injection (cleaner approach)

asciinema/theme/inject.js

@@ -1,22 +0,0 @@
-// Inject custom theme CSS and favicon
-(function() {
-  // Inject custom CSS
-  var link = document.createElement('link');
-  link.rel = 'stylesheet';
-  link.href = '/theme/custom.css';
-  document.head.appendChild(link);
-
-  // Inject custom favicon
-  var favicon = document.createElement('link');
-  favicon.rel = 'icon';
-  favicon.type = 'image/svg+xml';
-  favicon.href = '/theme/favicon.svg';
-
-  // Remove existing favicons first
-  var existingFavicons = document.querySelectorAll('link[rel*="icon"]');
-  existingFavicons.forEach(function(el) {
-    el.remove();
-  });
-
-  document.head.appendChild(favicon);
-})();