Sharing a Thermograph URL (Discord, Slack, iMessage…) now unfurls into a
card with the site name, page title, description, accent color, and logo.
- All five pages get description/theme-color/og:*/twitter:card meta and a
favicon. og:url/og:image need absolute URLs and crawlers don't run JS, so
pages carry an __ORIGIN__ placeholder the server fills in per request from
X-Forwarded-Proto (Caddy) / scheme + the Host header + base path — correct
on both the LAN dev server and the prod domain without hardcoding either.
- New logo assets: the header's ▚ mark as an app icon (accent quadrants on
the dark surface) — logo.png (512x512, the og:image) and logo.svg (favicon).
- Page routes render the substitution with a weak ETag + 304 revalidation
(replacing plain FileResponse) and now answer HEAD, which preview crawlers
probe with (previously 404).
