refactor: Decompose Timeline.tsx (1442 lines)

[ChuxiJ]

Description

src/components/timeline/Timeline.tsx is 1,442 lines acting as a mega-container for track rendering, scroll sync, keyboard navigation, playhead, and marker management.

Proposed Extraction

• useTimelineKeyboard.ts — Extract 200+ line keyboard handler
• useTimelineScroll.ts — Extract scroll sync logic
• PlayheadOverlay.tsx — Playhead positioning and rendering
• ArrangementMarkers.tsx — Loop markers, location markers

Acceptance Criteria

• Timeline.tsx reduced to < 500 lines
• Keyboard navigation fully preserved
• Scroll sync works identically
• All existing tests pass