Replace external Prism theme dependencies with local files

- Download prism-okaidia.min.css theme for dark mode
- Update setPrismTheme function to use local theme files
- Remove dependency on external CDN for theme switching
- Improve application self-containment and reliability

Author: codebude

public/ext/prism-okaidia.min.css

@@ -17,9 +17,9 @@ document.addEventListener('DOMContentLoaded', () => {
     function setPrismTheme(isDark) {
         const prismLink = document.getElementById('prism-theme');
         if (isDark) {
-            prismLink.href = 'https://cdnjs.cloudflare.com/ajax/libs/prism/1.29.0/themes/prism-okaidia.min.css';
+            prismLink.href = 'ext/prism-okaidia.min.css';
         } else {
-            prismLink.href = 'https://cdnjs.cloudflare.com/ajax/libs/prism/1.29.0/themes/prism.min.css';
+            prismLink.href = 'ext/prism.min.css';
         }
     }