fix: apply configured day length to weeks and years

[vycdev2]

Summary

• derive short-duration week and year units from todo.hoursPerDay
• preserve the existing 24-hour default while making custom day lengths consistent across day, week, and year output
• add regression coverage and document the user-visible fix

Verification

• npm run check — passed: typecheck, 402 tests, webpack development build, and formatting
• git diff --check — passed

Risk

• Low: the change is isolated to short-duration formatting when todo.hoursPerDay is customized; the default output is unchanged.

Fixes #16

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