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>

package.json

@@ -9,12 +9,15 @@
     "preview": "astro preview"
   },
   "dependencies": {
+    "@fontsource/jetbrains-mono": "^5.3.0",
+    "@resvg/resvg-js": "^2.6.2",
     "astro": "^5.0.0",
     "github-slugger": "^2.0.0",
     "katex": "^0.16.10",
     "mermaid": "^11.0.0",
     "rehype-katex": "^7.0.0",
     "remark-math": "^6.0.0",
+    "satori": "^0.28.2",
     "unist-util-visit": "^5.0.0"
   },
   "allowScripts": {