Build stamp, live listening strip, OG cards, closer shiki ground

- footer: 'built <ts> · <hash> · <n> notes', hash linking to Gitea
  (GARDEN_COMMIT passed in by build.sh; '+' marks dirty-tree builds)
- homepage header: live strip fed by api.c0smere.net/spotify/summary
  (weekly aggregate + monthly top artist/genre), fail-closed like the
  odometer and header quote
- /og/<slug>.png satori+resvg cards for every note + the homepage;
  og:/twitter: meta in Base
- shiki github-dark-default (#0d1117 sits closer to the page ground)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/pages/[...slug].astro

@@ -72,6 +72,7 @@ const contentLeadsWithTitle =
   title={title}
   description={entry?.data.description}
   headings={rendered?.headings ?? []}
+  ogSlug={entry ? slugForId(entry.id) : undefined}
 >
   {!contentLeadsWithTitle && <h1>{title}</h1>}
   {Content && <Content />}