fix: mount custom.css to assets directory, not css

static_paths only includes: assets, fonts, images, js
The css directory is NOT served by Phoenix.

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

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

asciinema/theme/custom.js

@@ -3,7 +3,7 @@
   // Inject custom CSS
   var customCSS = document.createElement('link');
   customCSS.rel = 'stylesheet';
-  customCSS.href = '/css/custom.css';
+  customCSS.href = '/assets/custom.css';
   document.head.appendChild(customCSS);
 
   // Inject custom favicon