Advanced export and audio conversion options

[thedavidweng]

Problem

Export is limited to copying the raw output file. No format conversion, no metadata tagging, no batch export presets.

Scope

• Audio format conversion (WAV → MP3, FLAC, OGG) via Symphonia or FFmpeg sidecar
• Metadata tagging (title, artist, genre from prompt/lyrics)
• Export presets (e.g., "Podcast quality", "Lossless archive")
• CLI openloop files convert <input> --format <fmt>

Acceptance criteria

• Users can convert generated WAV to MP3/FLAC/OGG from the UI
• Exported files include metadata from the generation record
• CLI openloop files convert supports format selection
• Conversion happens locally (no external service)