Implemented Phase 11.3 - Export Regions:
- Added export scope selector in ExportDialog
- Entire Project: Mix all tracks into single file
- Selected Region: Export only the selected region (disabled if no selection)
- Individual Tracks: Export each track as separate file
- Updated ExportSettings interface with scope property
- Refactored handleExport to support all three export modes:
- Project mode: Mix all tracks (existing behavior)
- Selection mode: Extract selection from all tracks and mix
- Tracks mode: Loop through tracks and export separately with sanitized filenames
- Added hasSelection prop to ExportDialog to enable/disable selection option
- Created helper function convertAndDownload to reduce code duplication
- Selection mode uses extractBufferSegment for precise region extraction
- Track names are sanitized for filenames (remove special characters)
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Co-Authored-By: Claude <noreply@anthropic.com>