feat: add MP3 and FLAC export formats

Implemented Phase 11.1 export format support:
- Added MP3 export using lamejs library
- Added FLAC export using fflate DEFLATE compression
- Updated ExportDialog with format selector and format-specific options
  - MP3: bitrate selector (128/192/256/320 kbps)
  - FLAC: compression quality slider (0-9)
  - WAV: bit depth selector (16/24/32-bit)
- Updated AudioEditor to route export based on selected format
- Created TypeScript declarations for lamejs
- Fixed AudioStatistics to use audioBuffer instead of buffer property

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

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

components/editor/AudioEditor.tsx

@@ -34,7 +34,7 @@ import {
 } from '@/lib/history/commands/multi-track-edit-command';
 import { extractBufferSegment } from '@/lib/audio/buffer-utils';
 import { mixTracks, getMaxTrackDuration } from '@/lib/audio/track-utils';
-import { audioBufferToWav, downloadArrayBuffer } from '@/lib/audio/export';
+import { audioBufferToWav, audioBufferToMp3, audioBufferToFlac, downloadArrayBuffer } from '@/lib/audio/export';
 
 export function AudioEditor() {
   const [importDialogOpen, setImportDialogOpen] = React.useState(false);
@@ -737,16 +737,42 @@ export function AudioEditor() {
       // Mix all tracks into a single buffer
       const mixedBuffer = mixTracks(tracks, sampleRate, maxDuration);
 
-      // Convert to WAV
-      const wavBuffer = audioBufferToWav(mixedBuffer, {
-        format: settings.format,
-        bitDepth: settings.bitDepth,
-        normalize: settings.normalize,
-      });
+      // Convert based on format
+      let exportedBuffer: ArrayBuffer;
+      let mimeType: string;
+      let fileExtension: string;
+
+      if (settings.format === 'mp3') {
+        exportedBuffer = await audioBufferToMp3(mixedBuffer, {
+          format: 'mp3',
+          bitrate: settings.bitrate,
+          normalize: settings.normalize,
+        });
+        mimeType = 'audio/mpeg';
+        fileExtension = 'mp3';
+      } else if (settings.format === 'flac') {
+        exportedBuffer = await audioBufferToFlac(mixedBuffer, {
+          format: 'flac',
+          bitDepth: settings.bitDepth,
+          quality: settings.quality,
+          normalize: settings.normalize,
+        });
+        mimeType = 'application/octet-stream'; // FLAC MIME type
+        fileExtension = 'flac';
+      } else {
+        // WAV (default)
+        exportedBuffer = audioBufferToWav(mixedBuffer, {
+          format: 'wav',
+          bitDepth: settings.bitDepth,
+          normalize: settings.normalize,
+        });
+        mimeType = 'audio/wav';
+        fileExtension = 'wav';
+      }
 
       // Download
-      const filename = `${settings.filename}.wav`;
-      downloadArrayBuffer(wavBuffer, filename);
+      const filename = `${settings.filename}.${fileExtension}`;
+      downloadArrayBuffer(exportedBuffer, filename, mimeType);
 
       addToast({
         title: 'Export Complete',