- Replace manual metadata extraction with regex in pyqt/conversion.py
with calls to domain/metadata_extraction.py functions
- Remove duplicate _embed_m4b_metadata and _apply_m4b_chapters_with_mutagen
functions from webui/conversion_runner.py
- Use ExportService.embed_m4b_metadata for m4b metadata embedding
- Reduce code duplication between PyQt and WebUI interfaces
