Add per-day notes for ranged todos

[Peace-Min]

Summary

• Add per-day detail notes for ranged todos while keeping the existing overall todo note
• Expose ranged todo end date in quick-add and show range labels in todo rows
• Persist todo day notes through XML import/export and clean notes outside the active date range

Tests

• npm.cmd test
• dotnet build widget\TaskCalendarWidget.csproj

[Peace-Min]

내용을 현재 main에 반영 완료(재구현, 커밋 b0297a6 — 기간 할일 날짜별 노트). 병합 대신 재반영했습니다. 감사합니다.