Export getCurrentDay and improve current day calculation for championship header

[dai]

Motivation

• Ensure the championship header (優勝争い) reflects the correct day when day 14 has started by deriving the current day from multiple data sources instead of a single published flag.

Description

• Add and export getCurrentDay in app/page.tsx which computes the current basho day using resultDays with status === 'published', a resultUpdatedAt reference date to consider isoDate-bounded result days, and today.makuuchi.day, then returns the maximum of those values.
• Update the Home component to use the new getCurrentDay result for currentDay and the showChampionshipRace flag.
• Update app/page.test.tsx to import getCurrentDay and add a test shows championship header as day 14 when day 14 has started that asserts the championship headings use the computed day.

Testing

• Ran unit tests including app/page.test.tsx which exercised the new getCurrentDay behavior and the new championship header test, and they passed.

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Codex Task

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