fix: add CommonJS compatibility for lamejs dynamic import

Fixed MP3 export error by handling both default and named exports
from lamejs module to support CommonJS/ESM interop.

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

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

lib/audio/export.ts

@@ -136,8 +136,9 @@ export async function audioBufferToMp3(
   audioBuffer: AudioBuffer,
   options: ExportOptions = { format: 'mp3', bitrate: 192 }
 ): Promise<ArrayBuffer> {
-  // Dynamically import lamejs
-  const lamejs = await import('lamejs');
+  // Dynamically import lamejs - use default export for CommonJS compatibility
+  const lameModule = await import('lamejs');
+  const lamejs = lameModule.default || lameModule;
 
   const { bitrate = 192, normalize } = options;
   const numberOfChannels = Math.min(audioBuffer.numberOfChannels, 2); // MP3 supports max 2 channels