Jumoo Releases & changelog: header/footer redesign, issues-closed on activity, downloads footnote

Jumoo/releases: redesigned the site header/footer — small square Jumoo icon top-left with the nav right-aligned, footer trimmed to two links (releases/changelog) plus a wordmark logo, explainer text separated and italicized
Jumoo/releases: activity page now shows issues closed alongside issues opened (already tracked in engagement.json but unused) in the at-a-glance card, chart, and per-package table; dropped the "busiest month" card to keep the at-a-glance grid at 4 boxes
Jumoo/releases: stats page downloads counter starred, with a footnote noting it only covers core tracked packages — full total is on the Jumoo org on NuGet
KevinJump/changelog: mirrored the releases site's header/footer redesign — square icon top-left, right-aligned nav pointing at this site's own archive/about pages, footer trimmed to a single releases link plus wordmark logo