[WIP] feat: MIDI file import — drag .mid files onto timeline to create piano roll tracks

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original prompt

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This section details on the original issue you should resolve

<issue_title>feat: MIDI file import — drag .mid files onto timeline to create piano roll tracks</issue_title>
<issue_description>## Problem

MIDI file import is a standard DAW feature that every competitor supports. Users need to import .mid files from other DAWs, online MIDI libraries, and external tools. Currently ACE-Step only has MIDI export planned (#215) but no import capability.

Competitive Reference

All competitors (BandLab, Amped Studio, Audiotool, Ableton, Logic, FL Studio) support MIDI file import.

Proposed Solution

User Flow

1. Drag & drop: Drag .mid file onto timeline → parse → create piano roll tracks
2. File picker: Import button / menu → select .mid file → same result
3. Multi-track MIDI: Each MIDI track/channel creates a separate piano roll track
4. Metadata: Import tempo, time signature, track names from MIDI file

Technical Details

• Use midi-parser-js or @tonejs/midi (MIT) for parsing Standard MIDI Files
• Map MIDI channels to piano roll tracks
• Convert MIDI ticks to beat positions using tempo map
• Handle both Type 0 (single track) and Type 1 (multi-track) MIDI files
• Respect note-on velocity mapping to our 0-1 velocity range

Store Integration

• New action: importMidiFile(file: File) → parses and creates tracks + clips
• Reuse existing addTrack('pianoroll') and addMidiNote() actions

Acceptance Criteria

• Drag .mid file onto timeline creates piano roll track(s)
• File picker import works
• Multi-track MIDI creates multiple tracks
• Note pitch, velocity, duration correctly mapped
• Tempo and time signature from MIDI file applied (with user confirmation)
• Track names from MIDI file used
• Unit tests for MIDI parsing
• E2E test for import flow

Related

• feat: MIDI export — export MIDI clips as .mid files #215 (MIDI export — complement to this feature)</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes feat: MIDI file import — drag .mid files onto timeline to create piano roll tracks #231

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[ChuxiJ]

Closing Copilot WIP — Claude Code/Codex taking over.