refactor: Decompose PianoRollCanvas.tsx (1391 lines)

[ChuxiJ]

Description

src/components/pianoroll/PianoRollCanvas.tsx is 1,391 lines combining canvas rendering with a complex mouse state machine for 5 edit modes.

Proposed Extraction

• usePianoRollDrag.ts — Custom hook for drag state machine
• PianoRollRenderer.ts — Pure canvas drawing functions
• VelocityLaneRenderer.ts — Velocity lane drawing

Acceptance Criteria

• PianoRollCanvas.tsx reduced to < 400 lines
• All 5 edit modes work identically
• Unit tests for drag state transitions