[Feature Request] MIDI-mappable step buttons in Beat+Bassline Editor

### Enhancement Summary

Currently, the step buttons in the Beat+Bassline Editor cannot be controlled via MIDI. Users with pad controllers (like the M-VAVE SMK25-II, Akai MPD, Novation Launchpad) have no way to toggle steps on/off in real-time. This limits live performance and beat-making without a mouse.

### Implementation Details / Mockup

Make each step button a MIDI-mappable target, similar to how knobs and faders are already mappable via right-click → "Connect to controller." A MIDI CC value > 64 would turn the step ON, < 64 would turn it OFF. Alternatively, MIDI Note On/Off could toggle the step.

### Please search the issue tracker for existing feature requests before submitting your own.

- [x] I have searched all existing issues and confirmed that this is not a duplicate.

Start by locating the Beat+Bassline Editor step-button implementation and the existing right-click "Connect to controller" handling for knobs and faders. Compare how MIDI-mappable targets receive controller input, then clarify whether steps should use CC thresholds or MIDI Note On/Off before defining tests for turning steps on and off in real time.

Written by the indexing model from the issue text.