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>

astro.config.mjs

@@ -13,7 +13,8 @@ export default defineConfig({
   markdown: {
     remarkPlugins: [[remarkObsidian, { slugMap }], remarkMath],
     rehypePlugins: [rehypeKatex],
-    // single dark theme — the site is fixed-dark to match blog.c0smere.net
-    shikiConfig: { theme: 'github-dark' },
+    // single dark theme — the site is fixed-dark to match blog.c0smere.net;
+    // -default variant's #0d1117 ground sits closer to the page's #0b0d12
+    shikiConfig: { theme: 'github-dark-default' },
   },
 });