refactor: decompose handleDragEnd into focused sub-handlers (#64)

[ryota-murakami]

Summary

• Extract recordColumnHistory() helper to deduplicate 3 identical undo-history blocks
• Extract handleNewRowDrop() for column → new row zone drops
• Extract handleColumnInsertDrop() for column → empty grid cell drops
• Extract handleColumnSwap() for column → column swap drops
• handleDragEnd is now a ~60-line dispatcher (was 285 lines)

All helpers use useCallback for stable references. No behavioral changes.

Closes #64

Test plan

• pnpm test — all 1202 tests pass
• pnpm typecheck — clean
• pnpm lint — clean
• pnpm e2e — verify DnD operations work correctly

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✨ Finishing touches

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• Commit unit tests in branch refactor/issue-64-decompose-handle-drag-end

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 13.63636% with 57 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.63%. Comparing base (1eb53df) to head (9b79cde).

Files with missing lines	Patch %	Lines
src/components/Board/KanbanBoard.tsx	13.63%	57 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #107      +/-   ##
==========================================
+ Coverage   74.46%   74.63%   +0.17%     
==========================================
  Files         120      120              
  Lines        3881     3880       -1     
  Branches     1063     1035      -28     
==========================================
+ Hits         2890     2896       +6     
+ Misses        968      961       -7     
  Partials       23       23              

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🧪 E2E Coverage Report (Sharded: 12 parallel jobs)

Metric	Coverage
Lines	94.38%
Functions	17.66%
Branches	16.89%
Statements	30.48%

📊 Full report available in workflow artifacts