Advanced export options (bitrate, codec, container)

[maboa]

Background

Once multiple formats are available (MP3/Opus and video export), power users will want control over bitrate, codec, and container details — e.g., choosing AAC vs. Opus inside an MP4 container, or higher MP3 bitrate for archive-quality audio.

Proposed change

Add an "Advanced settings" disclosure section to the export modal (same pattern as the rabbit modal's advanced toggle). Defaults stay sensible; advanced users can override.

Acceptance criteria

• Disclosure section in the export modal, collapsed by default.
• Bitrate input where applicable per format.
• Codec choices grouped under each container.
• Sample-rate / channel-count controls if useful.
• Defaults outside Advanced match what a non-advanced user would want.

Notes

Phase-3 nice-to-have once the basic export modal and format support land.