feat(site): visualize dataset growth history

[Seungpyo1007]

Summary

• replace the simple recent-sync list with a growth timeline chart
• replay recent public dump commits from site/public/v1/index.json
• show cumulative record totals and per-sync positive or negative deltas
• put History before Playground in the nav and hero CTA flow
• keep the current snapshot visible even if GitHub history is unavailable
• update PR metadata automation to avoid closing persistent tracker issues and mark project items in progress

Tracking

• Related to Track homepage and site improvements #19

Verification

• npm.cmd run build
• git diff --check