Cookieless, env-gated page-view tracking via Umami. The <Analytics />
component injects the script only when NEXT_PUBLIC_UMAMI_SRC and
NEXT_PUBLIC_UMAMI_WEBSITE_ID are both set, so local dev and forks send
nothing to our instance. Adds .env.example docs (section 6) and a
homepage footer privacy disclosure. No Cookie consent banner needed.
Zonghao Yuan