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>

types/lamejs.d.ts

@@ -0,0 +1,7 @@
+declare module 'lamejs' {
+  export class Mp3Encoder {
+    constructor(channels: number, sampleRate: number, bitrate: number);
+    encodeBuffer(left: Int16Array, right: Int16Array): Int8Array;
+    flush(): Int8Array;
+  }
+}