activeSessionSlice extensions:
- Add availableMixdowns array to store mixdown objects
- Add activeMixdown for currently selected mixdown
- Add mixdownCache for package metadata
- Add 4 reducers: setAvailableMixdowns, setActiveMixdown, cacheMixdownPackage, clearMixdowns
- Add 3 selectors for mixdown state
sessionUISlice extensions:
- Add openJamTrack to track currently open JamTrack ID
- Add jamTrackUI for user preferences (lastUsedMixdownId, volume)
- Add 3 reducers: setOpenJamTrack, updateJamTrackUI, clearOpenJamTrack
- Add 2 selectors for JamTrack UI state
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>