Refactor: Extract useKanbanDnD hook from KanbanBoard (God Component decomposition)

[ryota-murakami]

Background

Code quality analysis (2026-02-20) identified KanbanBoard as a God Component — 553 lines with 3 distinct concerns mixed together. This is the highest-priority refactoring item.

Current State

src/components/Board/KanbanBoard.tsx contains ~200 lines of DnD logic:

• Sensor configuration (Mouse, Touch, Keyboard)
• Custom collision detection (forgivingCollisionDetection)
• handleDragStart / handleDragEnd (~80 lines, deeply nested)
• Column operation sub-handlers: handleNewRowDrop, handleColumnInsertDrop, handleColumnSwap
• Grid dimension calculations, insertion zone calculations
• activeId / activeDragType state

Proposed Change

Extract a useKanbanDnD hook:

// src/hooks/board/useKanbanDnD.ts
const {
  sensors, activeId, activeDragType, insertionZones,
  handleDragStart, handleDragEnd,
} = useKanbanDnD({ statuses, cards, dispatch, recordColumnHistory })

Acceptance Criteria

• New src/hooks/board/useKanbanDnD.ts with explicit TypeScript interfaces (Params + Return)
• All returned functions wrapped in useCallback
• KanbanBoard reduced by ~200 lines (rendering + wiring only)
• No behavior changes — pure refactoring
• All existing tests pass (pnpm test && pnpm build && pnpm e2e:parallel)

Impact

• KanbanBoard: 553 → ~350 lines
• DnD logic becomes independently testable
• Follows the established pattern from BoardPageClient hook extraction